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After a couple days of measuring, and comparing every reading I could, gleened from the blueprints, and from other makers bolts, both G.I ( all manufacturers ) and commercial, I have deduced that it is time to announce my findings. Once again I am happy to say that they favor 762MM Firearms.

Like any undertaking, this was not without it's Oh nooo,
moments. Those times when you think something is headed south on you, and you give a large sigh. But then you realize, it's nothing, or something stupid you did to cause it, or didn't do. My first was when I tried to install the 762mm firing pin in the bolt supplied. I had been all over this bolt for two days, I have inspected every crevice, nook and cranny, so I knew it inside and out. When the pin wouldn't fit I just knew it was out of spec.
The pin that is, so I started inspecting it. After measuring and comparing everything about it to known G.I. I determined that the pin was fine. I went back to the firing pin hole dimensions on the drawings and took another look inside with my bore light. The bolt had been all over my bench, and moved several
times during the process, and had inadvertantly picked up a piece of a spent primer anvil sliver, in the hole,it was covered with oil, so it stuck easily...sticky bomb. I flicked it out with a pick and the pin inserted as it should. I then checked the firing pin protrusion and found it to be in spec as well. I checked rotational movement and clearance, up and down, and side to side, at the tail, and all looked
good.

Bolt dimensions from 762mm when compared to specs. on the drawings were all dead on except, for one little machining angle. On the right side of the bolt body, on the outer edge of the fin, the drawings call for the step down back into the overall width to be a 30 degree angle + or - 2 degrees, the cut here had been accidently run in straight to a 90 to the body. Then upon closer inspection I saw the remainder of the finished angled cut up against the bolt body. I asked Chris about this and he laughed and said "yeah, I did that, I knew the minute it happened, that you would spot it." It is a prototype after all and has nothing to do with function, so it's
hardly worth worrying about. It is put there like all radius cuts and bevels on the drawings to give the bolt a finished look, and to keep any sharp corners from hanging up as a rifle breaks in. The drawings call for all corners to be rounded with a minimum of a .02 radius -.01 . This is a very small turn in the scheme of things, and once again is no problem for me on a prototype. It was there as evidenced by the remaining 30 degree bit of meat left next to the body, but was too proud on the outside edge, and Chris did this himself by hand before
going to heat treat because he caught it, then reprogrammed the CNC machine to do the rest properly.

Bolt overall length, outside dimensions, lugs, lug to cartridge seat distances, internals ports, shroud lift from cartridge face, bolt face diameter, lug mean-spread distance, bottom arc for hammer actuation, tail diameter, cartridge shroud diameter and bevels, and overall thickness, checked out to within allowable specs on the plans. Most were right on the measurement and few were .001-.002, again on the proud side as allowed.

The RH lug length from cartridge face calls for .620 -.003 before heat treat and .617-.002 after heat treat, 762mm bolt measured .617. It doesn't get any better than that. This is the most important measurement, along with overall length, and the total outside to outside measurement of the lugs. This width keeps the bolt centered in the rails of the receiver and keeps the roller from crowding the receiver when traveling rearward. All of these were double and triple checked by me. After all, I am the one that had to fire it.

Bolt roller stud location, and height, in relation to the bolt centerline datum, and to the right hand locking lug, are another area of primary importance. The 762MM bolt also held
tolerances in these areas. If it is too short, it will let the bolt roller hit the receiver, if it is too long, it will put undue pressure on the roller and the op rod and reduce life of all parts involved. The roller spring (clip) groove also had all radius turns and inside sharp shoulders as called for by spec. The radius locations are specifically called for and stated to not be allowable in the spring groove, in order to keep the spring in place. I found the dimensions to be very good and fitting of the spring to be normal. For anyone that has not had experience with working on these, I cannot stress enough,
how important this is to proper function and being able to even install one. It won't last long if not exactly like it should be.

A few words about the internals made by 762MM Firearms, the firing pin has been covered, and I also took a good bit
of time with the extractor spring, bearing, and ejector
assembly. The springs had the proper number of coils when
compared to my new in the wrap G.I. spare parts, and measured out well against these. The ejector also had the correct angled cuts on the face and fit the spring inside diameter very well. The extractor bearing was a bit tight in the
spring and required a fair amount of pressure to seat it all the way. Because it looked a bit swelled on the outside of the spring, I elected to remove it and emory it down some on the bearing shaft, then reinstalled. I brought this to Chris's attention. I then reblued and oiled it and moved on to the extractor.

The extractor supplied had all the earmarks, and was reminiscent of the early Garand extractors, right down to
the finish on it. I have several of the original M14 extractors and M1's to go by. The comparison test all showed
the 762MM to be very well made and only one dimensional difference was noted. This is the rake (or angle) of the cut
just below the extractor bearing seat hole. The 762MM was ever so slightly at a steeper angle and actually helped
when installing the extractor. The angle cut is there to match up with the bearing tip and compress it into position in the seat, so that the extractor may pass to final location. I find this angle difference to have no change on proper function. The seat location was exactly the same as the G.I. when measured from the top or the bottom of the extractor. The depth and diameter of the seat was the same as well. This is an area of concern in all rifles due to changes in the past by commercial producers. I am glad that 762MM has kept it to G.I. spec.

Finally I was ready to install the bolt and check the headspace. First I wanted to inspect lug contact on both sides
with no internals installed. Then I checked the face and shroud for receiver and/or barrel contact. I proceeded
with many cycles of closing and opening and studying the feel for it, and compared the look of the right hand locking lug when bottomed out on the receiver and in full battery. I then set the guages out and started at 1.630, and progressed, checking each guage as I went. The pictures below tell the story. The bolt headspaced to my 6-2009 SAI receiver at 1.632, and the next guage would not allow closure of the bolt. The 1.632 headspace guage showed no interference or clearance fit, and I would describe it as a transitional fit, with no
additional friction required to close the bolt. This to me means the headspace was exactly 1.632 and not 1.6315 I then decided to try it on other barreled receivers, 1 Norinco, 2 Fultons, 3 Armscorps, 3 762MM's, and one LRB.
1- Norinco,- 1.633
2- Fultons,- 1.631 & 1.631 friction fit
3-Armscorps,- 1.630,interference- lapping needed & 1.631. & 1.631
3- 762MM's Forged,- 2-G.I. barrels, & 1 SAI barrel- 1.630, 1.631, & 1.631 transitional
1- LRB.- 1.630 transitional

The bolt lug to receiver lug contact was fine on all checked with the exception of the Norinco, and would require
a slight bit of lapping at the right lug to gain contact solidly to the left with more bearing surface, this would of course lead to at least 1.634 headspace or greater. Still, I believe this bolt would make for another candidate for chicom conversions to American made bolts. The mean average lug contact of all tested was 30-35 percent.

After all this work, the range time was much welcomed, except for the fact that the wind was howling at 30mph, plus gust to 40 mph. It was very cold and the high was forecast for 47, ..we never saw 41, and the wind chill was like a knife. I am only glad I wasn't going for a destruction test like the military
because I doubt I could have lasted that long. I fired 55 rounds with this bolt installed in my SAI rifle, and did the majority of them rapid fire, to try and force a malfunction, or failure to extract. I used my loads, and some winchester
white box that was Nato surplus. If it were going to have a problem, it would have been with these. I saw no problems
of any kind and the groups under the circumstances and rapid fire, gave me no pause or concern. I rolled on the trigger
a couple times and ran off five rounds as fast as I could. I only fired one rested group of five, and it was enough to show
that the bolt was returning to the same place each time. The wind caused the group to be all about 3" off the bull at 100yds ! That is a little much from what I know about wind calling, but it was too cold to investigate further into my rear sights.
I had already decided to not change them from center zero for this test. Upon breakdown afterwards, I noticed no peening or damage to the bolt anywhere, and ran some measurements again, to confirm they were the same. The extractor seemed to do a fine job as well, the brass was in good condition and was not all ripped up at the casehead where the claw grabs it.
All in all, I would say it is another success for 762MM Firearms.

I cannot speak to, or attest to, the surface or core hardess ratings of this supplied bolt because I did not cut it at the military's prescribed lines for testing. I put my faith in the manufacturer and fired it as supplied, and I am still here to relay my experiences. But Chris did cut examples at correct marks, and sent them across the street from him for the brinell hardness test. The results were, sampled cross sectionally, areas of the bolts where tensile and sheer capabilities greatest stress, and surface and and core Rockwell hardness were centerline with military specifications, with overall surface depth of 12-18 thousandths.

The rear ear of the bolt, on the left had a more sweeping radius from the bolt body top, down to the transition at the side, than the compared G.I. bolts, and upon checking the prints, I found a mere .003 angle difference coming from the back of the bolt to the turn, I later discovered that this was a good thing, in that it did not hang up on the receiver, just forward of the clip guide mounting fixture, and directly above the bolt stop window. If a bolt is too proud in this measurement area, it will inhibit mating the lugs forward. I had an SAI receiver and a G.I. bolt that was causing me problems with headspace, and I could not get the bolt to go rearward to get contact up front, I then realized it was hanging up at the rear bolt ear, on the receiver. A quick pass with a diamond file allowed me to continue with headspacing. This geometric difference from G.I. to 762MM bolts, IMHO, is a good thing.
The finish on the bolt can be compared in the following pictures as well, with all other manufacturers bolts.

A big thank you goes out to Doc for supplying the other makers barreled actions to use in my review !....Hey, he got a free chamber cleaning out of the deal.DI5

Stay tuned for future reviews......I just received one of 762MM Flash Suppressors and it is really something.
 

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Great review Rip! Now I know you do something besides hound dogging the PX. Thanks for taking the time to work on this, I'm sure it will benefit a lot of members.
 

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Excellent evaluation. Thank you again for info that helps the rest of us looking for parts for future builds. It's really great that 7.62 is doing such good work and continues to improve their product.
 

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GREAT review! I cant wait to receive my rifles from 7.62mm.
 

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"After all this work, the range time was much welcomed, except for the fact that the wind was howling at 30mph, plus gust to 40 mph. It was very cold and the high was forecast for 47, ..we never saw 41, and the wind chill was like a knife."

You'll be OK there Cupcake GI2

Thanks for your time to evaluate and write a great review.
 

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Thanks for this evaluation! My mind is set as ease about using one for a Polytech rifle upgrade!
 

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Discussion Starter · #11 ·
"After all this work, the range time was much welcomed, except for the fact that the wind was howling at 30mph, plus gust to 40 mph. It was very cold and the high was forecast for 47, ..we never saw 41, and the wind chill was like a knife."

You'll be OK there Cupcake GI2

Thanks for your time to evaluate and write a great review.
Finally, someone picked up on the wit....haa..leave it to a New Hampshire native to pick it out.....who was the one standing in my yard the day before the test and said ? " oooo, it's cold out here", on their way back to Jackson, Hmm,....

to the rest of you, thanks for the support. haa haa...another finish comparison shot and the tools of the trade.
 

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Discussion Starter · #14 ·
For this partcular job, and pic, these are Pacific Tool and Guage, headspace guages that run from 1.630 all the way through 1.6340. The whole set is handy for me because I never know when someone brings in a rifle, exactly what the headspace is. It helps alot on trouble-shooting on different rifles and problems and bolt conversions.
 

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For this partcular job, and pic, these are Pacific Tool and Guage, headspace guages that run from 1.630 all the way through 1.6340. The whole set is handy for me because I never know when someone brings in a rifle, exactly what the headspace is. It helps alot on trouble-shooting on different rifles and problems and bolt conversions.

Thanks for the info. sir.

I'm learning a lot from this sight.

Parashooter
 

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Awesome write up! FRG1

Thanks for taking the time to share this awesome info. I've been delaying on my M25 build to see what these puppies had to offer.
 

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Nice write up RIP. It is good to see some informative and detailed info on these.
 
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