.

The 8-40tpi screw size is correct as it's much stronger than the normal 6-48tpi size.

The easiest way to properly locate the screw hole pattern on the top of the receiver would be to buy a rail for the Mossberg 500, clamp it onto the receiver and use it's holes as both a template and a drill bit guide.

I would strongly suggest you D/T a single hole using the rail, then secure the rail prior to D/T'ing the other 3 holes to preclude the rail from moving around when doing multiple holes.

.

 

.

The 8-40tpi screw size is correct as it's much stronger than the normal 6-48tpi size.

The easiest way to properly locate the screw hole pattern on the top of the receiver would be to buy a rail for the Mossberg 500, clamp it onto the receiver and use it's holes as both a template and a drill bit guide.

I would strongly suggest you D/T a single hole using the rail, then secure the rail prior to D/T'ing the other 3 holes to preclude the rail from moving around when doing multiple holes.

.

That's a good idea I'll definitely get a rail before I drill so I can use it as a template. I already own an 8-40 tap but it's not the kind meant for doing blind holes which is why I was wondering if the holes on the model 500 go all the way through or not?

 

I already own an 8-40 tap but it's not the kind meant for doing blind holes which is why I was wondering if the holes on the model 500 go all the way through or not ?

Since mounting a scope on a shotgun puts severe recoil pressure on the scope mount, besides the stronger 8-40 screws, the screw mount holes should be through-holes, to get as many threads as possible to hold the base in place.

.

 

I already own an 8-40 tap but it's not the kind meant for doing blind holes which is why I was wondering if the holes on the model 500 go all the way through or not ?

Since mounting a scope on a shotgun puts severe recoil pressure on the scope mount, besides the stronger 8-40 screws, the screw mount holes should be through-holes, to get as many threads as possible to hold the base in place.

.

Well that would make it easier on me too, I was just wondering if the factory 500s have blind holes or not, I was thinking maybe there would be an issue with the bolt riding across those open holes vs a smooth inside of the receiver.

 

There are no issues unless the bolts stick down and the bolt hits them, then, you get cycling issues.

MOST long guns that are threaded for scope mounts use through holes, not blinds.

 

There are no issues unless the bolts stick down and the bolt hits them, then, you get cycling issues.

MOST long guns that are threaded for scope mounts use through holes, not blinds.

OK sounds good that puts my mind at ease. I just looked at my 8-40 tap and man that seems so tiny. Wouldn't you think a larger bolt would be better? Since I'm doing it from scratch I could do any size I want, so you guys think bigger would be better? I have a 1/4-28 tap that I could use which to me seems like it would be way stronger. What do yall think?

 

Keep it at 8-40. They are plenty strong enough.

 

I have a 1/4-28 tap that I could use which to me seems like it would be way stronger.

What do yall think?

I think that, if you do that, the scope mount would have to be altered to accept the new size (holes enlarged & countersunk for the screw heads), screws with a proper head pattern (not thread size) found, and any future re-sale endangered because, if the buyer didn't want a scope, there are AFAIK no 1/4-28 plug screws available.

Don't omit applying thread locker (Loc-Tite) to the threads on both the screws and the receiver after de-greasing them.

.

 

I have a 1/4-28 tap that I could use which to me seems like it would be way stronger.

What do yall think?

I think that, if you do that, the scope mount would have to be altered to accept the new size (holes enlarged & countersunk for the screw heads), screws with a proper head pattern (not thread size) found, and any future re-sale endangered because, if the buyer didn't want a scope, there are AFAIK no 1/4-28 plug screws available.

Don't omit applying thread locker (Loc-Tite) to the threads on both the screws and the receiver after de-greasing them.

.

Well I could easily make some plug screws, you're right about having to modify the rail. Do you guys have an opinion on the Ncstar or utg rails? Are they good enough or should I go for a better brand?

10-4 on the loctite and degreaser

 

I have a 1/4-28 tap that I could use which to me seems like it would be way stronger. What do yall think?

Oh hell no, for a Laundry List of reasons.

 

I have a 1/4-28 tap that I could use which to me seems like it would be way stronger. What do yall think?

Oh hell no, for a Laundry List of reasons.

Are you gonna list any of those reasons?

 

I have a 1/4-28 tap that I could use which to me seems like it would be way stronger. What do yall think?

Oh hell no, for a Laundry List of reasons.

Are you gonna list any of those reasons?

Since I am not on my phone now, maybe.

Receivers are thin, every thread turn you get in the metal increases strength of the threaded joint.
Each thread on a 28-pitch screw takes .0357" of metal thickness, so if the receiver top was .070" thick you'd ALMOST get TWO threads to pull on it.
Each thread on a 40-pitch screw takes .025" of metal thickness, so if the receiver top was .070" thick You'd get a full additional thread (almost THREE threads) to pull on it.
Each thread on a 48-pitch screw takes .0208" of metal thickness, so if the receiver top was .070" thick You'd get half an additional thread (3 and a half threads) to pull on it.

If you have 1/4" of metal to screw down into, 28TPI would be OK, you don't have.

Fine threads have better strength in tension, through a thick OR thin material, than coarse threads.

Fine threads also have a larger "minor diameter" the core where the threads are cut, than a coarser thread, which adds shear strength to the fastener.

The only time "diameter" is of any use is for resistance from SHEAR.
Your primary stop of shear is the joint being TIGHT, NOT screw size.
Allow the scope mounts to get loose, they will do 1 of 2 things, shear off screws (no matter how thick they are) OR twist/rock in the threads, destroying the receiver threads.
You are 100 TIMES better off, to prevent screws shearing to put a smooth surface locker like Locktite 680 BETWEEN the mount and the receiver AND put Locktite 242 on the #4, 6, or 8, screw threads to prevent looseness.

You do not have a thick enough receiver to make 28TPI a good idea, and you, no matter what shells you use, do not have enough recoil to "rip out" or shear off, properly tight #6 or #8 screws, not with ANY scope of any imaginably realistic weight.
What's that Red Dot weigh?

A lot less than either of the scopes, on TALL mounts (see-through) that ride on 2 BAR's (a .308 and a .270 (heavy variable magnification scopes)) and THOSE have been served for 3 and 4 decades (rifles ages) by #6-48tpi screws, 4 of them.

I've had one mount shear off, once, because the screws were small (#6), countersunk, ALLEN-HEAD (made the bottom of the countersunk head almost tissuepaper thin at the base) AND 2 decades of corrosion (un-seen under the screwhead).

Snapped one mount off at the range zeroing for deer season.
One screw head, or both, popped loose while shooting, then sheared the 2 screws off at the receiver surface.
The un-caught scope whipping around now ripped out 3 of the 4 screws holding the front mount CAP on (the mount base screws held fine), the thrashing destroyed the scope internally.

 

Since I am not on my phone now, maybe.

Receivers are thin, every thread turn you get in the metal increases strength of the threaded joint.
Each thread on a 28-pitch screw takes .0357" of metal thickness, so if the receiver top was .070" thick you'd ALMOST get TWO threads to pull on it.
Each thread on a 40-pitch screw takes .025" of metal thickness, so if the receiver top was .070" thick You'd get a full additional thread (almost THREE threads) to pull on it.
Each thread on a 48-pitch screw takes .0208" of metal thickness, so if the receiver top was .070" thick You'd get half an additional thread (3 and a half threads) to pull on it.

If you have 1/4" of metal to screw down into, 28TPI would be OK, you don't have.

Fine threads have better strength in tension, through a thick OR thin material, than coarse threads.

Fine threads also have a larger "minor diameter" the core where the threads are cut, than a coarser thread, which adds shear strength to the fastener.

The only time "diameter" is of any use is for resistance from SHEAR.
Your primary stop of shear is the joint being TIGHT, NOT screw size.
Allow the scope mounts to get loose, they will do 1 of 2 things, shear off screws (no matter how thick they are) OR twist/rock in the threads, destroying the receiver threads.
You are 100 TIMES better off, to prevent screws shearing to put a smooth surface locker like Locktite 680 BETWEEN the mount and the receiver AND put Locktite 242 on the #4, 6, or 8, screw threads to prevent looseness.

You do not have a thick enough receiver to make 28TPI a good idea, and you, no matter what shells you use, do not have enough recoil to "rip out" or shear off, properly tight #6 or #8 screws, not with ANY scope of any imaginably realistic weight.
What's that Red Dot weigh?

A lot less than either of the scopes, on TALL mounts (see-through) that ride on 2 BAR's (a .308 and a .270 (heavy variable magnification scopes)) and THOSE have been served for 3 and 4 decades (rifles ages) by #6-48tpi screws, 4 of them.

I've had one mount shear off, once, because the screws were small (#6), countersunk, ALLEN-HEAD (made the bottom of the countersunk head almost tissuepaper thin at the base) AND 2 decades of corrosion (un-seen under the screwhead).

Snapped one mount off at the range zeroing for deer season.
One screw head, or both, popped loose while shooting, then sheared the 2 screws off at the receiver surface.
The un-caught scope whipping around now ripped out 3 of the 4 screws holding the front mount CAP on (the mount base screws held fine), the thrashing destroyed the scope internally.

Hey Stuffer, I know I'm late to this discussion but do you think I'd have any issues going with an 8-36 instead? I work at a machine shop and can get my receiver D/T with CNC. We normally stock 8-36 screws and taps so I'm curious if that would be a valid alternative. Also wondering if I'd have issues mounting rails. I assume it would be the same since they're both 8s but want to make sure beforehand. (I'm a young engineer, not a machinist in case you're wondering why I have limited knowledge on this).

 

Since I am not on my phone now, maybe.

Receivers are thin, every thread turn you get in the metal increases strength of the threaded joint.
Each thread on a 28-pitch screw takes .0357" of metal thickness, so if the receiver top was .070" thick you'd ALMOST get TWO threads to pull on it.
Each thread on a 40-pitch screw takes .025" of metal thickness, so if the receiver top was .070" thick You'd get a full additional thread (almost THREE threads) to pull on it.
Each thread on a 48-pitch screw takes .0208" of metal thickness, so if the receiver top was .070" thick You'd get half an additional thread (3 and a half threads) to pull on it.

If you have 1/4" of metal to screw down into, 28TPI would be OK, you don't have.

Fine threads have better strength in tension, through a thick OR thin material, than coarse threads.

Fine threads also have a larger "minor diameter" the core where the threads are cut, than a coarser thread, which adds shear strength to the fastener.

The only time "diameter" is of any use is for resistance from SHEAR.
Your primary stop of shear is the joint being TIGHT, NOT screw size.
Allow the scope mounts to get loose, they will do 1 of 2 things, shear off screws (no matter how thick they are) OR twist/rock in the threads, destroying the receiver threads.
You are 100 TIMES better off, to prevent screws shearing to put a smooth surface locker like Locktite 680 BETWEEN the mount and the receiver AND put Locktite 242 on the #4, 6, or 8, screw threads to prevent looseness.

You do not have a thick enough receiver to make 28TPI a good idea, and you, no matter what shells you use, do not have enough recoil to "rip out" or shear off, properly tight #6 or #8 screws, not with ANY scope of any imaginably realistic weight.
What's that Red Dot weigh?

A lot less than either of the scopes, on TALL mounts (see-through) that ride on 2 BAR's (a .308 and a .270 (heavy variable magnification scopes)) and THOSE have been served for 3 and 4 decades (rifles ages) by #6-48tpi screws, 4 of them.

I've had one mount shear off, once, because the screws were small (#6), countersunk, ALLEN-HEAD (made the bottom of the countersunk head almost tissuepaper thin at the base) AND 2 decades of corrosion (un-seen under the screwhead).

Snapped one mount off at the range zeroing for deer season.
One screw head, or both, popped loose while shooting, then sheared the 2 screws off at the receiver surface.
The un-caught scope whipping around now ripped out 3 of the 4 screws holding the front mount CAP on (the mount base screws held fine), the thrashing destroyed the scope internally.

Noted, I was thinking of shear because I had a ruger pistol that sheared off a bolt on the scope mount. Not sure what size bolt but it was a lightweight scope. The red dot won't be heavy but I'd also be using a 3-9x40 scope for slugs and the red dot for turkey hunting and 2 oz loads and looking at that tap just seemed like it'd be weak, but I didn't realize that hi powered rifles and such use #8 bolts. But 8-40 will make it easier anyway since I'll be able to use the bolts that come with the mount and not modify anything.
What do you think of the cheapy mounts like utg and Ncstar? Good enough or should I go for a better brand?

 

Since I am not on my phone now, maybe.

Receivers are thin, every thread turn you get in the metal increases strength of the threaded joint.
Each thread on a 28-pitch screw takes .0357" of metal thickness, so if the receiver top was .070" thick you'd ALMOST get TWO threads to pull on it.
Each thread on a 40-pitch screw takes .025" of metal thickness, so if the receiver top was .070" thick You'd get a full additional thread (almost THREE threads) to pull on it.
Each thread on a 48-pitch screw takes .0208" of metal thickness, so if the receiver top was .070" thick You'd get half an additional thread (3 and a half threads) to pull on it.

If you have 1/4" of metal to screw down into, 28TPI would be OK, you don't have.

Fine threads have better strength in tension, through a thick OR thin material, than coarse threads.

Fine threads also have a larger "minor diameter" the core where the threads are cut, than a coarser thread, which adds shear strength to the fastener.

The only time "diameter" is of any use is for resistance from SHEAR.
Your primary stop of shear is the joint being TIGHT, NOT screw size.
Allow the scope mounts to get loose, they will do 1 of 2 things, shear off screws (no matter how thick they are) OR twist/rock in the threads, destroying the receiver threads.
You are 100 TIMES better off, to prevent screws shearing to put a smooth surface locker like Locktite 680 BETWEEN the mount and the receiver AND put Locktite 242 on the #4, 6, or 8, screw threads to prevent looseness.

You do not have a thick enough receiver to make 28TPI a good idea, and you, no matter what shells you use, do not have enough recoil to "rip out" or shear off, properly tight #6 or #8 screws, not with ANY scope of any imaginably realistic weight.
What's that Red Dot weigh?

A lot less than either of the scopes, on TALL mounts (see-through) that ride on 2 BAR's (a .308 and a .270 (heavy variable magnification scopes)) and THOSE have been served for 3 and 4 decades (rifles ages) by #6-48tpi screws, 4 of them.

I've had one mount shear off, once, because the screws were small (#6), countersunk, ALLEN-HEAD (made the bottom of the countersunk head almost tissuepaper thin at the base) AND 2 decades of corrosion (un-seen under the screwhead).

Snapped one mount off at the range zeroing for deer season.
One screw head, or both, popped loose while shooting, then sheared the 2 screws off at the receiver surface.
The un-caught scope whipping around now ripped out 3 of the 4 screws holding the front mount CAP on (the mount base screws held fine), the thrashing destroyed the scope internally.

Noted, I was thinking of shear because I had a ruger pistol that sheared off a bolt on the scope mount. Not sure what size bolt but it was a lightweight scope. The red dot won't be heavy but I'd also be using a 3-9x40 scope for slugs and the red dot for turkey hunting and 2 oz loads and looking at that tap just seemed like it'd be weak, but I didn't realize that hi powered rifles and such use #8 bolts. But 8-40 will make it easier anyway since I'll be able to use the bolts that come with the mount and not modify anything.
What do you think of the cheapy mounts like utg and Ncstar? Good enough or should I go for a better brand?

Your The MAN!. Thank You for your in dept info and facts!
I hope he listens to you.

 

Noted, I was thinking of shear because I had a ruger pistol that sheared off a bolt on the scope mount. Not sure what size bolt but it was a lightweight scope. The red dot won't be heavy but I'd also be using a 3-9x40 scope for slugs and the red dot for turkey hunting and 2 oz loads and looking at that tap just seemed like it'd be weak, but I didn't realize that hi powered rifles and such use #8 bolts. But 8-40 will make it easier anyway since I'll be able to use the bolts that come with the mount and not modify anything.
What do you think of the cheapy mounts like utg and Ncstar? Good enough or should I go for a better brand?

I'm not real fond of UTG's single small allen-screw that clamps the mounts to the rail, looks to me like it could shift under recoil, probably be FINE for a .223AR (which is what it is really marketed at), they are basically recoilless rifles.
Can't find anything appropriate at NcStar to opine on.

As a general rule, I prefer steel scope mount bases.
Very often scope mounting on a shotgun forces you to canteliever out over the barrel, which is NOT the direction to go cheap and flimsy on.
Also shotguns (especially fixed breech guns) jolt a lot harder than AR15's do.

I also do not have an Unlimited Budget.

2 scopes I have recently mounted to M1913/Picatinny rails, were mounted using leupold's IMS (Integral Mounting System), for around $120-$140 apiece.
They went on a short-ranged Hog-Killing .308W and a 9-yr-old's AR (with a lot more scope than Hog's bane).

The third was a Warne, on a long-range .308W upper for serious glass (6x24 Nikkon).

All 3 of those are Aluminum, but with large steel bolts that cross-clamp them to the rail.

There are 100 other good, solid, won't-give-you-ANY-trouble scope mounts out there to go on Pic-Rails (it's the main way things are done today), but for a 'gauge, I wouldn't spend $20-30 on one.
Am .223R AR15 can get away with a lot of crap, but then too, a Range Toy or a Serious Hunting Gun, I look at them differently.

I'd drop at LEAST 3-4 times that, at a minimum.

 

Noted, I was thinking of shear because I had a ruger pistol that sheared off a bolt on the scope mount. Not sure what size bolt but it was a lightweight scope. The red dot won't be heavy but I'd also be using a 3-9x40 scope for slugs and the red dot for turkey hunting and 2 oz loads and looking at that tap just seemed like it'd be weak, but I didn't realize that hi powered rifles and such use #8 bolts. But 8-40 will make it easier anyway since I'll be able to use the bolts that come with the mount and not modify anything.
What do you think of the cheapy mounts like utg and Ncstar? Good enough or should I go for a better brand?

I'm not real fond of UTG's single small allen-screw that clamps the mounts to the rail, looks to me like it could shift under recoil, probably be FINE for a .223AR (which is what it is really marketed at), they are basically recoilless rifles.
Can't find anything appropriate at NcStar to opine on.

As a general rule, I prefer steel scope mount bases.
Very often scope mounting on a shotgun forces you to canteliever out over the barrel, which is NOT the direction to go cheap and flimsy on.
Also shotguns (especially fixed breech guns) jolt a lot harder than AR15's do.

I also do not have an Unlimited Budget.

2 scopes I have recently mounted to M1913/Picatinny rails, were mounted using leupold's IMS (Integral Mounting System), for around $120-$140 apiece.
They went on a short-ranged Hog-Killing .308W and a 9-yr-old's AR (with a lot more scope than Hog's bane).

The third was a Warne, on a long-range .308W upper for serious glass (6x24 Nikkon).

All 3 of those are Aluminum, but with large steel bolts that cross-clamp them to the rail.

There are 100 other good, solid, won't-give-you-ANY-trouble scope mounts out there to go on Pic-Rails (it's the main way things are done today), but for a 'gauge, I wouldn't spend $20-30 on one.
Am .223R AR15 can get away with a lot of crap, but then too, a Range Toy or a Serious Hunting Gun, I look at them differently.

I'd drop at LEAST 3-4 times that, at a minimum.

Sorry I should have been more clear I was talking about the rail itself. I have plenty of high quality scope rings and red dot mounts. Here are the 2 rails in question -
https://www.ncstar.com/optics-acc/mounts/shotgun-mounts/mshrcvmos-mntmos500rcv-railblk
http://leapers.com/index.php?act=prod_detail&allids=_&itemno=MNT-MB500T

 

Any opinions on these rails?

 

I can't imagine why you would want to use a 1/4 28 screw. Do you think you know more than the engineers that design this stuff? You would have a hard time shearing the 8-40 screws with a hammer. I spent over 40 years as a machinist so I have some experience with screw strength. IMO

 

This thread is 2 years old pal

 

Not exactly related, but instead of screwing it in could that be soldered in place?

 

There are saddle mounts that do not require drilling, that use the trigger pack pins. Would need to check but most 500 accessories work on the 88. WEAVER NO-GUNSMITH CONVERTA SHOTGUN MOUNTS | Brownells