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 Post subject: LMT AR, 7.62 X 51 ammunition and chambering decision - help
New postPosted: September 15th, 2015, 8:17 am 
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I have a LMT modular rifle set up with a 7.62 X 51 barrel. Have had it for over 2 years and it is a tack driver with Mil Surp 7.62 X 51 NATO 147 gr.

Could not be happier until I just recently decided to try some Federal 168 gr. Gold Medal Match in .308 (generally reserved for my bolt gun). Gun was well lubed (run them wet) and warmed up. It fired the round and extracted / ejected just fine. The next round chambered (guessing not completely) and thankfully did not fire out of battery. I could not extract the round as it seemed locked up solid. Had to bunker it and beat it into disassembly. So,
I again cleaned and lubed the rifle and promptly repeated the fiasco on the first shot. HMMM, WTF I say as I again beat the rifle apart. Nothing shows on the brass case, no burr in the chamber. Projectile shows no deformation from the chambering process. So, back to basics - I dropped a NATO round into the chamber (with no bolt in rifle) and turn gun muzzle up; round falls out freely. I repeat the same motion with the 168 gr. round and it sticks firmly in the chamber (cold bore both tests). These 168 gr. are awesome in my M40A1 bolt gun so I believe the rounds are well within spec.
Up until now I had believed that the .308 would run fine in the 7.62 X 51 chamber. I do not own a go / no go gauge for this caliber. Not so much in this rifle evidently.

Seriously need some suggestions on this problem. I'd like to be able to run some of the 168 and 175 gr. fodder available for 7.62 X 51. I am somewhat fearful that opening up the throat will kill the excellent accuracy of the 147 gr. Mil Surp. LMT was very helpful on the phone but I'd like to avoid sending it off for service vs. seeing a local smith if that is what you all think is indicated.

Thanks for reading, thoughts and suggestions.

Chris

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 Post subject: Re: LMT AR, 7.62 X 51 ammunition and chambering decision - h
New postPosted: September 15th, 2015, 5:14 pm 
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Hmmm,... I have found that not all weapons shoot all manufacture types of ammo. This may be a case of your AR just not able to use the Fed ammo.

I would take a mic/caliper and get serious about comparing EVERY DIMENSION of both your surplus and the Fed match ammo. I would assume that OAL is correct or your mags would have let you know of that problem.


good luck,
8)

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Last edited by leverman on August 23rd, 2016, 4:27 am, edited 1 time in total.

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 Post subject: Re: LMT AR, 7.62 X 51 ammunition and chambering decision - h
New postPosted: September 16th, 2015, 2:27 am 
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I had the same problem with the Fed. 168gr. with a M1. The ogive on match ammo is different than standard 308. What was happening is the bullet is hitting the rifling and getting stuck making it hard to extract.


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 Post subject: Re: LMT AR, 7.62 X 51 ammunition and chambering decision - h
New postPosted: September 16th, 2015, 2:38 am 
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CR wrote:
I had the same problem with the Fed. 168gr. with a M1. The ogive on match ammo is different than standard 308. What was happening is the bullet is hitting the rifling and getting stuck making it hard to extract.


Yeah that!!!

Take one of your factory rounds liberally coat the bullet with a black felt tip or some other marker and chamber it. You will likely see the bright copper showing through the black coating where it touches the lands of the barrel before going into battery. Try the same thing in your bolt gun.

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 Post subject: Re: LMT AR, 7.62 X 51 ammunition and chambering decision - h
New postPosted: September 16th, 2015, 6:40 am 
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I would get a case gauge, highly doubtful that the projectile is hitting the lands and locking up the bolt, just doesn't happen unless you have supercrimp. Typically on this, when the factory case was sized, it was done incorrectly. The same happens on reloaded ammo when resized wrong. A case gauge will show this. Once the ram makes contact with the sizing die, lower ram and turn another 1/4" down on the sizing die, if this is not done cases will get stuck in the chamber, as well as on improperly trimmed brass, and yet again a case gauge will show this. I see this same issue all the time with 308 ARs.

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 Post subject: Re: LMT AR, 7.62 X 51 ammunition and chambering decision - h
New postPosted: September 16th, 2015, 1:37 pm 
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elheffe wrote:
I would get a case gauge, highly doubtful that the projectile is hitting the lands and locking up the bolt, just doesn't happen unless you have supercrimp. Typically on this, when the factory case was sized, it was done incorrectly. The same happens on reloaded ammo when resized wrong. A case gauge will show this. Once the ram makes contact with the sizing die, lower ram and turn another 1/4" down on the sizing die, if this is not done cases will get stuck in the chamber, as well as on improperly trimmed brass, and yet again a case gauge will show this. I see this same issue all the time with 308 ARs.


Yes, no marks at all on the projectile suggesting ogive is not touching lands and if rammed into chamber I'd expect extraction and a spilled powder mess. These Federals run great in the M40A1 bolt gun so resizing seems kind of a double edged sword. A concern I have is if I start seating these factory rounds deeper then I will increase pressure.

I know that match rounds I load for single shot are extra long but not relevant to this magazine fed platform (currently).

Rather than resizing the Federal Match rounds would you guys think that Milsurp Match ammo spec at 7.62 X 51 in a 168gr or 175 gr. may work? In other words is it more likely case dimension / OAL from the .308 Match or a LMT chamber sizing issue?

Pure speculation without dimensional data but hoping for some experienced consensus.

Next question - which smiths in the valley can finalize diagnosis and who do you trust?

Thanks,

Chris

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 Post subject: Re: LMT AR, 7.62 X 51 ammunition and chambering decision - h
New postPosted: September 16th, 2015, 5:09 pm 
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Never had an issue with oal other than fitting in the magazine, it's always case neck length or sized wrong. Don't really know anyone who works on 308 ar's I have just built quite a few and have been loading 308 for 15 years. That's my only experience with them

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 Post subject: Re: LMT AR, 7.62 X 51 ammunition and chambering decision - h
New postPosted: September 18th, 2015, 2:58 am 
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I would expect just the opposite result. It can't be an OAL problem if they fit in the magazine and feed. The profile of a 762 vs 308 says a 308 should work in a 762 but the opposite would push the shoulder back some. See the following.

http://i.ytimg.com/vi/uYlte9Z9Eqk/maxresdefault.jpg

This is talking about a FAL but has a section on chamber size difference of 762 vs 308, which is .013

http://www.303british.com/id36.html

I have both DPMS and POF 762s I will see what brand of commercial 308 I have for my bolt gun and see they chamber in the ARs when I get back from work.

If you want, you can drop one round of the Federal off and I will try to chamber it in my ARs also. That may narrow it down to the rifle or the ammo.

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 Post subject: Re: LMT AR, 7.62 X 51 ammunition and chambering decision - h
New postPosted: September 18th, 2015, 4:22 pm 
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ynotaz wrote:
I would expect just the opposite result. It can't be an OAL problem if they fit in the magazine and feed. The profile of a 762 vs 308 says a 308 should work in a 762 but the opposite would push the shoulder back some. See the following.

http://i.ytimg.com/vi/uYlte9Z9Eqk/maxresdefault.jpg

This is talking about a FAL but has a section on chamber size difference of 762 vs 308, which is .013

http://www.303british.com/id36.html

I have both DPMS and POF 762s I will see what brand of commercial 308 I have for my bolt gun and see they chamber in the ARs when I get back from work.

If you want, you can drop one round of the Federal off and I will try to chamber it in my ARs also. That may narrow it down to the rifle or the ammo.


Ynotaz, thank-you for those links. It baffles me as everything I have researched has said it should feed fine. I know LMT builds a tight chamber, they told me so when we spoke. I'd happily relegate all the Fed. Match to my bolt gun but need to find a heavier weight that works well for distance.

Sincerely appreciate your offer to check your two rifles for me and happy to give you some Fed. Match to compare. Much respect.

Chris

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 Post subject: Re: LMT AR, 7.62 X 51 ammunition and chambering decision - h
New postPosted: September 19th, 2015, 3:21 am 
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Maybe I missed it, are the Fed Match factory new or reloaded for your bolt gun? If reloaded was a full length or small base die used? I use a small base die for all may semi auto guns. Just trying to get to figure out the problem.


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 Post subject: Re: LMT AR, 7.62 X 51 ammunition and chambering decision - h
New postPosted: September 19th, 2015, 10:52 am 
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Since the OP says Federal Premium I assume it is factory ammo and not reloads.

Now on to the bad news.....

Image

I found some Federal 180 Grain soft point in the back of my Bolt Gun stash. I pulled the uppers off the lowers to test. The round dropped into the chamber of both the DPMS and POF rifles. The bolt closed with the little more pressure than the weight of the bolt and charging handle. The slight extra effort was only required to overcome the extractor/ejector springs. Both guns extracted and ejected easily.

I'd be happy to trade a couple 180gr for a couple of your 168grs to see if the ammo is out of spec. or very different profile.

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 Post subject: Re: LMT AR, 7.62 X 51 ammunition and chambering decision - h
New postPosted: September 19th, 2015, 4:01 pm 
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Yes, appreciate the offer to swap some factory Fed Match 168. Perhaps bring my LMT upper along? Got your PM. Will Comm. Check and Lv. Msg.

Thanks again,

Chris

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 Post subject: Re: LMT AR, 7.62 X 51 ammunition and chambering decision - h
New postPosted: September 19th, 2015, 5:08 pm 
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If timing works, yep we can have all 3 rifles to see what is going on.

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