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LBR Receiver & USGI NM Barrel Installation

1559 Views 10 Replies 9 Participants Last post by  hammonje
I am purchasing a LBR receiver and will be sending my NOS USGI bolt and barrel to them for installation.

The barrel is marked MFG 26978 9345206 6-83 82472. It also has “NM” at the end of the muzzle and “P” towards the chamber. I think I have read most of the posts about NM barrels. It appears this is a General Dynamics / Saco barrel manufactured in June 1983 using drawing number 9345206, yes? Also I would like to know the twist rate.

I am wondering if LBR will have any particular problems installing this barrel? I read Gus Fishers post about these barrels having” very long” headspace. What do I need to tell LBR regarding this potential problem and can it be fixed. If I need a different barrel this is the time to get it.

Thanks for your help!
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Your best bet is to simply ask the Gentlemen at LRB ........ I have not found a more knowledgeable group.

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Also I would like to know the twist rate.
here is a way to determine ROT
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Second on LRB knowing what they are doing. If he is available, speak to Paul for believe he is the one that would most likely be doing the work or can point you in the right direction. LRB offers a barrel that is supposedly a copy of the one you mention.
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The medium weight General Dynamics barrels have a history of shooting very well. They are 1-12" twist which is fine for up to and including the 175 Sierra MK.

I wouldn't worry too much about headspace yet. It is affected by your receiver, your bolt, and your barrel. Arrange for LRB to contact you and give you an estimate if you are worried. They will be able to estimate the HS of your particular combination of parts before they torque the barrel on.

What kind of bolt do you have? The NOS TRW bolts are rumored to have been machined long, at the very upper limit of tolerances, so they tend to give shorter HS than other make bolts or used bolts.

Do you have a link to Gus Fisher's post about HS with the General Dynamics barrels? I tried a search but Gus has so many posts about HS I couldn't dig it out of the haystack.
I read Fisher's post/comments which is why I am concerned. Thanks for your insights.
I had a new barrel from Saco (in a tube) just like the one you describe above. If this barrel is new and has never been installed, you should not have an issue with headspace on an LRB. What I would be most interested in is the neck and throat dimensions.

The last one I installed was incredibly accurate, but I did have an earlier one that was reamed by an unknown person and the neck was oversized and the throat was long. It had way too much jump even with 175s.

I would recommend you cast the chamber. Anytime you go in with a pull through reamer and adjust headspace, the neck and throat can be cut non-conentric if the pilot and or the neck on the pull through is undersized. It can also wobble as it cuts and cut the body portion of the chamber even though the body is not supposed to be cut.

If this barrel is NOS; it is far too expensive and historically valuable to go willy nilly with just any pull through reamer.

If you take a cast, you can specify pilot diameter, neck and lead for the reamer that is used to cut the chamber. It probably has at least a .3445" neck diameter.

If the barrel is long on headspace, as in 1.637 + .002". I would venture to say that it has been previously installed and finish reamed.

USGI receivers tend to be .004" longer than most commercial receivers to include the stacking tolerance of the receiver thread timing and setoff, receiver face and bolt lug abutments. These barrels were made to be used on USGI receivers and therefore will have a shorter chamber HS value; measured from the barrel threads to barrel shoulder junction. Having a shorter HS value means it will be short on commercial receivers, which are typically .004" tighter. Often overlooked is the thread timing. Some measure the distance from the receiver face to the bolt lug abutment and use that as their value, but this value can be maligned by both receiver and barrel thread timing and thread offset.

Hope this helps.
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And thats why Jon Wolfe is building my XM25!


Thank you for your thoughtful insights. The barrel has no obvious signs of prior installation, no blemishes to the finish, no thread wear, etc., but I am not a gunsmith. The person from whom I purchased the barrel knew its history and its source.

Is there a particular receiver brand and/or bolt that would work better with a used barrel that may be long on headspace?
SAI receivers tend to provide more generous HS in my experience. I'm speaking about the older ones, pre-2004. I haven't used any others beyond 155k. No idea about contemporary geometry.

Most other commercial receivers I've used are shorter, but my experience is limited to the number of receivers below. I probably have built on the order of 50 M1s and M14s so far. Hope to add another zero on the end of it before my days are over.
JRA - 6+
LRB - 3
FA - 20+

TRW bolts in general seem to provide the shortest HS. When I am attempting to shorten HS with a combo I grab a NOS TRW bolt. Just to buck that trend I have an GI SA bolt that provides even shorter HS. Go figure. Best thing to do if you do not have a pile of GI bolts is to use commercial barrels. If I had only one bolt I would not purchase a GI barrel as it is too much of a crap shoot.
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