none concentric comp affect accuracy?

[jingerale]

ok so I was too cheap to buy a 1/2-28 9mm comp. I had a 5.56 comp and bored it out to 3/8" to clear a 9mm. I used a mill and thought I had it centered pretty well.

So here's the result, I took some rough measurements with a caliper. I'm pretty sure it'll clear the bullet. But it seems a little off center, not by much. What does the forum say? will it affect accuracy? It's going on a 16" Faxon barrel.

[baih777]

Thats why they make reamers.

[jingerale]

Thats why they make reamers.

well, i don't have any reamers, trying to do this on the cheap.

[GartenZwerge]

bore it bigger... but centered

[CoopsDad]

Shoot it and see. It's a 9mm AR. You're not hoping for single MOA accuracy anyway so it'll almost certainly be just fine.

[musketjon]

A reamer won't center the hole. A reamer smooths up a drilled hole and follows the hole. If it's off-center, putting a reamer through it, you will still have an off-center hole. If you used a mill, why didn't you indicate it in? .030" is enough room to drive a freight train through. Put it back in the mill, INDICATE IT IN (and check the squareness of the head to the table as well--indicate it too, if necessary--don't believe the hash marks on the head and the ram) and use a carbide (not high speed) end mill of appropriate size, OR a boring head and do the job correctly. Sounds like a text book case of not enough time to do it, but plenty of time to do it over.
Jon

[Cokebottle]

^^^^

If it's off center, but drilled to 9mm (3/8 is only 0.5mm over) then you probably are going to shave a little copper off.

[Barbarosa]

bore it bigger... but centered

This /\/\/\/\

[SilentPea]

Find someone with a lathe and buy them a 6-pack.

[sonofeugene]

Find someone with a lathe and buy them a 6-pack.

Using a lathe makes the assumption that the original ID and OD were concentric.

OP, you really don't care about the relationship between the ID and the OD. You need to be concerned about the relationship between the ID and the threads. You could nail concentricity between ID and OD and if the threads are off, the ID will still be off relative to the gun bore. And, of course, if the threads on the barrel are off.......

[sonofeugene]

I just threw your numbers up on CAD and your new bore center has shifted up and to the right by 0.212mm or 0.0083". If one assumes that when the comp is attached to the barrel and that the OD of the comp is perfectly concentric to the gun bore, then with a 9.01mm projectile OD, you'll be whizzing by the lower left wall of your comp by 0.133mm or 0.0052".

Of course, things are never perfectly concentric, so frankly, I'd redo your bore job, making sure it's as concentric as possible and only just cleaning up things as necessary. By my calculation, a new, perfectly centered bore in your comp at 0.399" would do it.

[SigLover556]

Science!!

[FaxonFirearms]

It can cause an issue, but unlikely with the 9mm. The pressures behind the bullet at uncorking are so low, deflection will be kept to a minimum.

We'd say shoot it and only if you have an issue look to machine anything further.

[Dogbite]

Using a lathe makes the assumption that the original ID and OD were concentric.

OP, you really don't care about the relationship between the ID and the OD. You need to be concerned about the relationship between the ID and the threads. You could nail concentricity between ID and OD and if the threads are off, the ID will still be off relative to the gun bore. And, of course, if the threads on the barrel are off.......

100% correct

[baih777]

shoot it and see what happens. if that didn't work I would just buy one of these.

http://www.ebay.com/itm/TriDelta-Tactical-Custom-Muzzle-Brake-for-9mm-Pistol-Carbine-w-1-2-28-TPI-MB35-/162229174718?hash=item25c59cc1be:g:QJIAAOSwzaJX9Ys4