AR Sponsor
Posted: 2/26/2008 8:00:18 PM EDT
|
I just got a used upper off gunbroker. It's in perfect condition except for a slightly canted FSB (about 7 degrees right). The guy I bought it from doesn't take returns, so unfortunately that's not an option. I intended to use an EOtech on this upper, with a BUIS. Will the FSB be much of an issue? |
+1 The odds are about ten to one that the canted front sight assy is not a front sight problem; but rather a barrel that was installed by clamping the upper reciever in a vise, rather than the barrel; resulting in the barrel turning to the left as the nut was tightened. Result? Rifle shoots far to the right, and needs a bunch of left windage adjustment at the rear. Since you're going to use an Eotech, I guess you can leave it as it is. Me? I'd get the barrel remounted properly, so you can use properly adjusted irons if, as, and when. |
| The barrel has hardly any "play" when it is put into the receiver. The problem comes from the barrel not being accurately indexed at the factory. The indexing pin on the barrel extension is supposed to line up with the gas port under the front sight base. Some say "smack the crap out of the front sight base with a mallet". The problem with doing that is that the front sight base does double duty as a gas block. Moving the front sight base could potentially limit the amount of gas that makes it from the barrel to the bolt carrier. Excessive twisting in the receiver can be stopped by using shims intended for use with barrel installations, and/or grinding a very little bit of the receiver where it holds the index pin to bring it back over to the other side. |
That would have to be a pretty extreme case of FSB cant. |
+1 And besides, banging the FSB isn't intended to move the sight assy. It's meant to shift the whole barrel slighly. Remember, it only takes a couple of thousanths of shift at the barrel/upper junction to shift the point of impact 8-10 clicks. A properly dimensioned/installed front sight assy isn't going to be moved by a rap w/a leather or rubber hammer, but the barrel will rotate just a bit; and that's what you're after. 
|
AR Sponsor