How to Lap Bolt Lugs

AR/M4 AR Technical Discussion
1

I recently completed an SPR build, and I want to lap the bolt lugs, or at least determine how much contact exists.

Parts were purchased from G&R, BCM, Lilja, and JP Enterprises. Machine work by ADCO. Details: LT Stealth Upper; Lilja 20" 1/8 three groove bbl; JP bolt carrier and bolt; NF 5-22x56 scope on ADM SPR mount. Initial load workup using Lapua Brass, Federal Match primers, Vihta Vouri N140 powder, and 69gn Sierra MKHPBT and Hornady 75 gn HPBTM pills. Results @ 100 yds: best 5 shot group = 0.4" with 75gn Hornady in front of 24gn VVN140. Better results expected with less wind, finer load increments, and perhaps using Berger bullets. At this stage, the accuracy is encouraging and indicates that further tweaking of the rifle and loads is warranted.

BTW, the barrel exhibited zero copper fouling using Montana Xtreme Insanely Toxic bore cleaner (sold at all the finest cosmetics counters). First time I have used a 3 grove barrel, and I was impressed. Dan Lilja said it would foul less than a 6 groove. He was right.

I have lapped the bolt lugs on many bolt guns and M1A’s, but never an AR. I have searched the posts and found nothing relevant. If there is a relevant thread or post, please direct me to it.

In the case of a bolt action rifle, of course, the tried and true method of lapping the lugs to achieve full contact (or at least adequate contact) is to cut a piece of brass in half crosswise, and solder a stiff spring linking the two halves, with the spring keeping the two halves perhaps 3/8" apart. Apply Dykem to the lugs, followed by fine lapping compound, insert the modified case into the chamber, and then close/open the action to see where the lugs are contacting and how much.

The problem with the AR action is that the buffer spring would probably not be powerful enough to close the action. I suppose a rubber mallet and a wooden stick could be used to whack the BCG into battery.

Question 1: Any suggestions on a better way to lap the lugs on an AR?
Question 2: Any suggestions for how best to lock to bolt manually (without relying on the buffer spring)?

Of course, headspace changes will be monitored and acceptable limits will not be exceeded.

Thanks for your input

KYPD

2

I have tried find info about lapping lugs on a AR-15 and only thing I could find was that it is not needed. One accuracy enhancement I have found was lapping the upper receiver barrel flange to insure that it is true with the bolt race way and the barrel has full contact with the upper.
http://www.brownells.com/.aspx/pid=20220/Product/AR_15_M16_UPPER_RECEIVER_LAPPING_TOOL

3

An AR-15 is not a bolt gun and lapping the bolt lugs for an AR-15 is a waste of time. Also, 5-shot groups are statistically insignificant in determining the accuracy of a rifle/ammunition combination, however if that is where you place relevance, here is what a precision AR-15, without lapped bolt lugs can do.

4

Thanks. Have you used the tool you linked to?

5

No sir I have not but here is a link showing it being used .
http://www.ar15.com/forums/topic.html?b=3&f=4&t=412304&page=1

6

A very pretty five shoot group indeed. Thanks for showing it. What powder and pill works best for your rifle?

I read your posts on barfcom. Very nice.

http://www.ar15.com/forums/topic.html?b=3&f=118&t=279218&page=3

Your statement that “lapping the bolt lugs for an AR-15 is a waste of time” implies that uniform lug contact on the AR-15 is irrelevant to accuracy. I find that difficult to blindly accept, although I concede that the degree of improvement achievable with uniform contact would be less in the case of the AR than for a quality, trued bolt gun. What is your statement based on? Is it an opinion or is it based on experience? Have you done before/after testing, with the only changed variable being lapping the lugs? Or can you direct me to published results of such testing?

Thanks for the observation re statistics. I can only agree with what you say, but when working up loads, I’m sure you’ll agree that 5 shot strings are a quick and efficient method to determine general trends. Of course, the more rounds in a group, the more useful the SD, and the lessons learned. But if I had to do 50 shot strings for every likely case/powder/primer/bullet combination when working up loads for a new rifle, the barrel’s throat would be shot out before I could even finish working up loads, and the money spent on powder, primers, brass and lead would be tremendous. Not to mention the time spent on brass prep. Or are you telling me that you put that kind of effort into working up loads for your tack driver?

http://rds.yahoo.com/_ylt=A0oGkyKaTWZKbAMA3ytXNyoA;_ylu=X3oDMTByZWgwN285BHNlYwNzcgRwb3MDMQRjb2xvA3NrMQR2dGlkAw--/SIG=1327pt4pd/EXP=1248304922/**http%3A//www.shootersjournal.com/Features/Haps/EnglemanChronographStatistics.pdf

I read your posts on barfcom. Very nice.

http://www.ar15.com/forums/topic.html?b=3&f=118&t=279218&page=3

Any answers to the questions in my first post?

7

Thanks for the link. I will order the tool. LaRue Tactical says that their Stealth upper is trued, but a double check can’t hurt.

8

Larue is correct…

Its pointless to try to true a Larue Stealth upper…all you will do is remove the anodizing and find it was was a waste of time…

9

Thanks for your insight. Have you done this yourself, or have you seen it done by others?

10

allow me to suggest that you contact frank white at compass lake engineering,if ANYBODY knows best how to accurize an AR,it’s frank. good luck----

11

Excellent idea. Thank you. I will do so.

12

Following the advice of responders, I checked with JP Precision and Compass Lake Engineering.

The consensus was that, so long as high-quality parts are being used, lapping lugs does not help, and is not done by either firm. Both companies build tons of highly accurate rifles every year, so their opinion carries great weight.

However, this is counterintuitive, since uniform bolt lug contact has been absolutely proven to improve accuracy in bolt guns. The AR15 has a bolt, which supports the case head. So it would seem that it would benefit also. Perhaps the difference is the number of lugs? Or perhaps the intrinsic eccentric loading on the lugs makes lapping an insignificant improvement? Or perhaps there are so many other out-of-control factors in the AR15 design not found in a bolt gun that makes AR15 bolt lug contact insignificant?

Anyway, thanks for everyone’s input. I shall experiment.