M40A1 USMC spec clip slot drawing.

[JRFirearms]

Hi all, i’m a rifle builder from the UK and i’ve been tasked with adding the clip/lug slot features and thumb slot on some E serial number actions. I’m not sure how accurate the builds will be but i’d like to get my part as accurate as possible.

I’ve been reading a lot of great information on SH but i can’t quite find what i need. As i understand it the original M40A1’s were retrofits from M40’s, and these actions had the true clip slot and double radius thumb slot of the 40X and machined by Remington. I have a technical drawing for this and it has enough details to get fairly close.

Then it appears any later supplied actions were machined in-house by the USMC. And these were machined with a simpler “lug slot” and single radius thumb slot. I can’t seem to find any drawings or technical specs for these features. If you look at the attached images i found on google it appears official drawings do exist and i wondered if anybody had a copy they would share? Or at least share specifications so i can get it close. Unfortunately in the UK there is little information.

Any help or input is greatly appreciated.

 

[JRFirearms]

Still no joy finding the full official USMC M40A1 lug slot drawing, but i did manage to get an official M40A3/5 lug slot drawing.

All i can find are a couple images of actions that appear to have been done the same way as the partial technical drawings i found.

 

[Random Guy]

As i understand it the original M40A1’s were retrofits from M40’s, and these actions had the true clip slot and double radius thumb slot of the 40X and machined by Remington. I have a technical drawing for this and it has enough details to get fairly close.

One must remember that the M40A1 was effectively hand-made from 1977 to about 2001, and there are lots of small variations of the build - given they pre-dated contemporary CNC machines. I have seen a variety of muzzle crowns on vintage M40A1s, etc.

In addition, it should be understood that the old Unertl scope mounts were basically hand-fit to the actions via hand-filing of the scope mount's rear lug. As two retired USMC MOS 2112s who built M40A1s in the 1990s explained to me years ago, the rear lug on the scope mount was designed to be slightly long, and the 2112s would tediously hand-file the rear face of that lug so it barely fit into the receiver. (See below arrow).

Anyhow, I think the drawings you have are sufficient, but if you want to be perfect, the M40A1 rear lug slot should be cut perhaps 0.002"(?) shorter than the distance between the front lug and the rear surface of the rear lug on the scope mount. That way you too can also tediously hand-file the rear lug so it has an almost 'zero clearance' fit that requires light pressure to install the scope mount into that lug slot. That is what was done with my build, which was built by one of those retired 2112s who made them back in the day...

(Fwiw, if you want to get another detail right, you can stamp the last 4 digits of the serial # on (edit) the top or bottom of the scope mount - since the hand-fit scope mount was supposed to stay married to the same M40A1 receiver during its service life....)

Did I mention that various aspects of the old M40A1 rifles were hand-made before the modern era of precision CNC machines?

 

[JRFirearms]

Hi Random guy, i appreciate your input. My research has come to the same conclusion. And yes, the reality is there was likely alot of varience. It just would of been nice to see a proper drawing from the 90’s before they went to the A3.

Yours is another design i’ve seen done a few times, it’s like the original M40 clip slot but they have just made the pocket a little larger. Which makes sense as it would be easier to machine and allow the corners of the lugs to have clearance due to the radius.

It would be great to have some input from some 2112’s to see if they worked to any spec or just done it how the wanted.

 

[cplnorton]

Ok, so I have good news and bad news. The good news is I have the actual PWS clip slot Machinist drawings from the M40A1 build guide from the 1990's. I found it at the National Archives. The bad news is I gave my word I will not share the actual drawing. So if you have a question on a measurement, I can help confirm or deny numbers. I just can't in good conscience give out a copy, as I gave my word I wouldn't. You are welcome to PM me on it though if you would like.

On the actual M40A1 Clip slot (the one that everyone calls the PWS style) that style did not originate till the 1990's. The reason being before the 1990's, all M40A1's were just rebuilt M40 receivers. The original M40 receivers were factory clip slotted by Remington in the 60's, so there was no need for a PWS clip slot modifcation. It wasn't till the Marines started to buy new receivers in the 1990's that the new style clip slot came about.

Also just a comment, I have not seen any Unertl Mounts with the last four of the serial stamped on the bottom of the mount. I'm not saying it couldn't happen, but all evidence I have seen shows they were marked on the top.

The whole numbering of the Unertl mounts seems to be a late modification. Most of the original mounts I have seen, even ones still on M40A1's in Marine inventory were not marked with the last four. It is also not mentioned in the Marines docs till the mid 1990's. Even interviewing several 2112's who were there for the whole M40A1 build program from the beginning till the end, they said it only came around late.

So if you are building one, unless you are going for a later style build, I would not personally mark the mount.

Now if you want to mark it, the Marine manual on building the M40A1 from the 1990's states, " Stamp the last 4 digits of the weapon serial number onto the top frame of the mount centered."

For instance, here is an original mount that was likely marked when serial 221,240 was rebuilt in 1998.

 

[cplnorton]

Yours is another design i’ve seen done a few times, it’s like the original M40 clip slot but they have just made the pocket a little larger. Which makes sense as it would be easier to machine and allow the corners of the lugs to have clearance due to the radius.

It would be great to have some input from some 2112’s to see if they worked to any spec or just done it how the wanted.

They had machinist drawings that gave the exact specs for the PWS Clip Slot to the receiver, but you are correct that the original M40 Remington Clip Slot was just a fuzz wider especially in the rear.

The Marines detailed when you fit a New Unertl Mount to an original M40 receiver, you only had to hand fit the rear lug in the back, like what Random guy shows above.

In contrast the Marine build guide details on the replacement receivers, you might have to file the sides on the Unertl mount as well to get them to fit.

The receivers were cut to spec, to the drawings. But the Unertl mounts were hand fit to the receiver for a tight fit.

 

[JRFirearms]

Hi CplNorton, great information, thank you. I totally understand and appreciate the situation. I will drop you a PM about some details i have to see if i’m on the right track.

 

[Random Guy]

Hi all, i’m a rifle builder from the UK and i’ve been tasked with adding the clip/lug slot features and thumb slot on some E serial number actions. I’
not sure how accurate the builds will be but i’d like to get my part as accurate as possible.

While we are on the subject, I"ll quickly share a odd little variance on a real USMC E-prefix M40A1 - note the crude/hand-inscribed "US" above the serial number...I have seen this more on the 1992 era C-prefix M700s, but I guess a 2112 decided to inscribe that "US" on this particular E-prefix rifle. I don't recommend you do that, but just an observation about these rifles. It has the 'RTE-P' proof stamp on the barrel, which is something clone builders often do on these retro builds to make them look more "correct".

This E-prefix had the later "PWS-P" stamp, and also the barrel install date stamped on the left side (month + 2 digit year). This rifle did not have the "US" hand engraved, so lot of small variances were seen...even black buttpads on a few M40A1 stocks. Anyhow, this was a VERY late M40A1, as I recall the barrel date year was "00" or "01" (2000 or 2001), and it had a US Optics ("USO") made scope, which were replacement scopes for the small number of Unertls scopes that were not-repairable. I recall that contract award to US Optics was back in 2000, so this M40A1 was likely one of the last M40A1 builds before PWS re-built them into the M40A3 configuration.

...Perhaps the funniest variance that I saw on real M40A1s was a rifle with a "--SUPER GRADE--" M70 bottom metal. I guess pre-64 bottom metal was hard to find, and a 2112 used whatever was available, including what was once a highly polished magazine plate on a high-end Winchester Model 70 hunting rifle...but I digress.

 

[cplnorton]

Random Guy makes some really good points and honestly, each era of the M40A1 build program was a little different. It just depends on which era you want to build it to.

The "official" M40A1 build guide is from the 1990's, when the shop was still the RTE shop and they were working on the C prefix or Charlie receivers. In that book it details they are supposed to inscribe the US mark on the receiver (as seen by Random Guy), and then mark the last four of the serial on the Bolt, trigger guard, floorplate, stock, and the mount. Then mark the barrel with the RTE proof mark. The original builds in 1977 had variations to the above.

The E, or Echo receivers came in after the shop became the PWS shop circa 1999ish. There is not an "official" manual I have seen for the PWS changeover and E receivers, but I have seen a lot of the E "Echo" M40A1 builds. I've also seen a lot of small notes here and there on the changes in the builds in the Marine docs for these receivers. There were a lot of variations in the Echo Receivers with the markings. Some barrels were dated, but then it was determined to be redundant because the rifle book said the same info. Some Unertl mounts were marked with the serial, most weren't. Some barrels were hand-stamped with single stamps to write out the PWS-P as the proof mark, some had the later PWS stamp after they had one single stamp made that said PWS.

I think the reason there was so much variation in the Echo receivers was because the Marines were already done with the M40a1 by that point. They were already focused on the M40A3 and working out the flaws in that rifle system and the M40A1's were only being made because they needed rifles and the M40A3 wasn't fully operational yet. So the Echo M40A1's were built after they already knew the M40A1 was basically done and they were switching to a new design. The Echo receivers were just a temporary fix to a dying rifle platform. I think that is why there is so much inconsistency in them. You also see a lot of recycled parts in these.

On a side note, looking at all the M40A1 serial numbers, and M40A3 serial numbers, I don't really see any M40A1's that were converted to M40A3. But that is also detailed in the work orders and Marine docs. I know Ken Davis had told me initially they wanted to keep the M40A1 and M40A3 programs separate and start with all new receivers on the M40A3 program and that is the same thing I see in the Marine Docs as well. The work order to build all the new M40A3's, was followed up with a contract with Remington to buy all new receivers.

One final thought. By the end the M40A1 program, it became a program of cannibalization. They would break down rifles for parts to keep other rifles running. So that can create a lot of variation for all the different eras of the M40A1 build program.

So when I was examining all the M40A1's in Marine inventory, you would often see some random part that seemed to not match up to the era. Even though there were guidelines to what was correct on these builds, Marines are Marines and parts are parts. We as collectors like to clone them perfectly to what our knowledge of them is, but the Marines were only worried if they could shoot accurately and if they functioned or not. I did see a lot of recycled parts in the builds.

 

[Basic user]

I'm not tracking. Sniper rifles are a type of assault rifle and people in the UK cant have those, AND tell us we're all crazy for even wanting them so... what are YOU doing? Does your government know about this?

 

[smoothy8500]

I'm not tracking.

Obviously....

 

[JRFirearms]

I'm not tracking. Sniper rifles are a type of assault rifle and people in the UK cant have those, AND tell us we're all crazy for even wanting them so... what are YOU doing? Does your government know about this?

I think you’re mis-informed. The term assault rifle doesn’t really exist here. We have section 1 firearms which covers all bolt action rifles of any calibre, .22LR semi-auto, long barreled pistols (minimum 12” barrel) and semi-auto shotguns with more than 2 shot capacity. Then there is Section 2 which is all 2 shot capacity or less shotguns. And finally we have section 5, this covers semi and full auto centrefire calibres, stun guns, short barreled pistols etc.

Section 1 and 2 are readily available with a license which includes background checks etc. section 5 can be owned with special permission. This is generally only pistols.

 

[JRFirearms]

Thanks for all the input, it makes for good research. I find it just as interesting as actually building them.

 

[macethetemplar]

 

[sinister]

Merry Christmas. Note the scope base and iron sight base holes are the original 6-48 and not the naval-military 8-40. M40XBs, 40Xs (both centerfire and rimfire), and M24s had the iron sight base holes on the aft-left of the receiver (700s were NOT drilled and tapped):

What goes in the left side iron sight holes (Redfield Olympic sight base. After Redfield went out of business they used RPA bases and sights until the end of the M24 program):

For RPA Trakker:

 

[JRFirearms]

That appears fo be the full version of the cropped version i found

 

[JRFirearms]

Merry Christmas:

View attachment 8651327

Thank you, i have this drawing. It’s the remingron M40/40X vs the M40A1 just posted above.

 

[cplnorton]

The one Sinister shared is the original M40, Remington drawing. The ones Mace shared is from the PWS build guide for the M40A1.

I'm glad it will help people and I don't have to break my word about not sharing it.