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Posted: 12/19/2011 5:12:13 PM EDT
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First time lapping an upper; not sure which compound or type of compound to use. I found the following on Brownell's and was wondering if this is what I needed:
http://www.brownells.com/.aspx/pid=1137/psize=24/Product/LAPPING-COMPOUNDS TIA for the advise/responses. |
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I used VersaChem 13209, available at the local auto parts house. There's two tubes, coarse and fine. Fine isn't slow, it quickly dresses the entire nose if the drill driver speed is up. It only takes a few revolutions, and check it, you wind up wiping off most of it to see the progress.
Most users see a few thousandths squaring up, it's really minimal on good quality uppers. The custom builders charge extra and the honest ones have reported it's not even part of their procedure. Apparently all it can do is center up the windage and elevation on a barrel, but it doesn't necessarily reduce the group size. Ammo and optic make a much bigger improvement. For an upper that's cranked all the way over, it's needed, seems to be one in a hundred. Not to forget, while its facing the nose, so it the shoulder on the tool. I got a nice ring on mine, it'll take turning on a lathe to true up again. Compound works both ways. |
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Yes, this process centers the windage and elevation - nice benefit. But, the primary thing it does is square the barrel to the upper - That is the HUGE gain. This in turn makes a square transistion of the bolt to the barrel extension. Finally, square transitions make the rifle slightly more accurate and much more RELIABLE. If you constantly smack the side of the bolt against the a piece of steel, it will eventually break. Does all of this make sense?
This is the correct thing to do to an upper. |
Is there a good thread describing and showing the process of how this is done? Is this something that is recommended even for a factory built upper like a BCM, which I have? Sorry for the noob question, but this is the first I have read of lapping the upper, and would like to understand more about the process.
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I have done all two dozen of my ARs this way. I use a 600 grit AO compound to see how bad out the front is from square. And ALL of mine have been out some, even the billet machined VLTORs. If it is just a little out, I continue with the 600 grit until square. If it is a LOT out, I switch over to Clover 325 grit SC compound. That makes short work of the process but check frequently because it takes it off in a hurry and you don't want to remove any more off the front than to get it square. Directions say 80% but IMO if you don't go all the way, why bother? I go until I just get 100% coverage then quit.
This is what I have found. Zero difference in reliability. Zero difference in accuracy. No difference in longevity from what I can tell, but none of my rifles have more than 10K through them so they may be too new to tell anything and I have never had anything break. The BIG difference I see it is that when I am done and zeroed, both scopes and irons are very near the middle of their adjustment. I have had several that before squaring had the sights cranked all the way over to one side or the other, and after squaring the sights zero very close to their center position. While some may say it does not matter so long as it is zeroed, I disagree. If you are shooting long range and the wind kicks up and you want to crank in real windage instead of Kentucky windage, having close to full range of adjustment sure comes in handy. |
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Depending on the length of sight radius, barrel, and what your choice in forearm is, lapping may be ideal. For my uppers that are under 16" I didn't find any real benefit to them. My LMT upper was perfect. My 16" Alexander Arms Grendel upper wasn't, and after lapping you could see a big difference. Consequently when I was lapping it the tool had a bit harder time turning than other receivers did. I believe any 16" upper I do for myself from here on out will get it as part of the build. Anything that mounts to my SBR won't since they're not going to be long range uppers. The thing I'll worry most about is concentric threads for my suppressor when I shoot for the first time.
Posted Via AR15.Com Mobile ETA: many 65 Grendel guys with broken bolts reported having some uppers break bolts and the culprit seemed to be an out of true receiver. Some were also using reloaded ammo which may have been too hotly charged. I found having the receiver true is an insurance policy/preventive maintenance. |
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Video better than words? FYI, this can be done with a drill press...I have heard of others doing it with a drill.
Video |
| Thanks for the comments. I was re-seating a barrel in an upper and noticed that barrel wobbles against the face; it can end up slanted to the receiver if the barrel isn't secured well when tightening the nut. It's not much that needs to come off the receiver. Also noticed that the FSB is slightly canted and requires approx. 9 clicks right windage to correct at 50. Probably exacerbated by the fact that the receiver and barrel aren't true. At this point, any little bit helps. |
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Quoted:
I used VersaChem 13209, available at the local auto parts house. There's two tubes, coarse and fine. Fine isn't slow, it quickly dresses the entire nose if the drill driver speed is up. It only takes a few revolutions, and check it, you wind up wiping off most of it to see the progress. Most users see a few thousandths squaring up, it's really minimal on good quality uppers. The custom builders charge extra and the honest ones have reported it's not even part of their procedure. Apparently all it can do is center up the windage and elevation on a barrel, but it doesn't necessarily reduce the group size. Ammo and optic make a much bigger improvement. For an upper that's cranked all the way over, it's needed, seems to be one in a hundred. Not to forget, while its facing the nose, so it the shoulder on the tool. I got a nice ring on mine, it'll take turning on a lathe to true up again. Compound works both ways. I'll look for the VersaChem - is it cheaper than the alum oxide stuff Brownell's sells? Does the VersaChem contain graphite?? |
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