Ok, so a fellow I know does a lot of his own work to his ARs, and in this regard he’s way ahead of me, so I figured I’d run some stuff he’s said recently by the folk here and see if I’m right or not in thinking there’s something a bit off…
So a while ago he was talking about zeroing a new AR he got. It had a fixed USGI-style front sight, as usual, and I think an ARMS #40 rear. Or it may have been some sort of fixed rear- I don’t remember 100% as this was all a month or so ago, but he likes the ARMS so it was probably that.
It would not zero correctly, as he said he turned the rear sight all the way to one side, and still could not get it on paper. I figured it was either a bent barrel, bad barrel alignment, or rear sight not on straight/out of spec upper- said so to him, but he said it was ok.
So he said he took the front sight, and gave it a couple wacks with a mallet in the direction it needed to go to get it to zero in. Apparently that’s how someone he knows does/did it in the Army…
Needless to say, I was like…:stop:
Is this legit?
I actually had this same type failure to zero issue with my DD, and it was a gun issue, not a FSB alignment issue…
Second thing was he installed a new FSB- one of those folding ARMS ones. He did the same “whack it into zero” with that too- according to him, the FSB pins did not engage the slots in the barrel…or maybe he said it was the slots were cut to shallow to engage- whichever it was, I was still enough of a “WTF” moment to have me scratching my head over it for the last month or so.
I spoke with an armorer years ago who said he had done this. If he had an AR that would not windage zero, he would lay the rifle on sand bags and whack the FSB with a soft lead mallet. I opted to buy tools to loosen the barrel nut to rectify the problem.
The front sight base needs to rotate in the opposite direction you want the bullet to impact on target. If I have a carbine that is shooting right with the rear sight adjusted all the way to the left, I loosen the barrel nut, insert a wood dowel rod in the front sight base and apply CCW pressure while I tighten the barrel nut. In most cases this has allowed me to adjust the rear sight to a usable part of the windage adjustment.
The AR receiver is made of alloy, not steel, so avoid using a BFH for making adjustments.
I encountered this on my first couple builds and a brand new rifle I just got. Some uppers have a wee bit of slop in the index pin slot. Since I am using an upper vise block I found upon tightening the uppers get turned to the “wrong” side of the slop in the slot. Since the GI Method has the barrel in a barrel vice block the upper does not shift. I had to run my rear Irons all the way left to hit paper. When I figured out what the deal was I made some tiny shims out of some splattered 44 mag bullet jackets. I put them to the right of the pin in the slot ( view is looking at the upper from the muzzle end ). I then applied the Molykote P37 paste and torqued until lined up. Reassembled and bamyo…mechanical zero right on with barely a couple clicks adjustment to make them dead nutz.
On the new rifle I just got I found in trying to zero it that some numbnutz whom installed the barrel did not even torque to 30 lbs. I disassembeled , cleaned and applied molykotte and reassembled. The index pin to slot fit was bugger tight !..no shim needed. Took 70lbs torque to line up. Zeroed with Irons with two clicks windage none on front sight. Removed Buis and zeroed ACOG like a walk in the park.
Lol. I have seen it done. But proper thing to do is drop a straightness gage to make sure it’s not the barrel and insure everything is lined up right. Take of the front sight post and realign it.
but like they say if that doesn’t fix it a BFH will or a tanker bar
PS ; I fixed a neighbors “olympic” junker with shims to the new Del-ton barrel he wanted on it. His was so out I had to file one side of the index pin slot and use a .'012" bronze shim to get sights on mech zero. But it worked and no BFH needed.
When using barrel vice blocks you are much more likely to have the upper shift than when using receiver vice blocks.
Using barrel blocks during assembly was the main reason for Old Bushmaster of Maine having so many canted FSBs.
Receiver blocks became popular because of this problem. Not that receiver blocks solved the problem completely, though it did make it less likely to happen.
In figuring this issue out by hands on …my buddies that use barrel vise blocks rarely encounter this. Think about that maybe the receivers are meant to index to the left side of the pin slot ( left side while looking at it from the front ) when torqued the index pin will naturally stay and stop to the left . When in an upper receiver block vise any slop has it shift right.
With two people it is easier, but I have done roughly 20 by myself.
With the receiver mounted in a vise block, I position the upper with the barrel on the bench side of my vise. I place the dowel in the front sight base and pivot the vise until the bottom of the dowel rod makes contact with the side of the bench, then rotate the vise/upper toward the bench until there is a bow in the dowel rod. This frees up both hands to engage the teeth of the barrel nut with the barrel nut wrench/torque wrench.
One thing I have noticed over the years is that after I get about 8 ft.lbs of torque on the barrel nut, tension on the dowel rod is no longer necessary.
I want my rifle windage at mechanical zero after zeroing the rifle on the range on a no wind day. In almost all cases I have been able to do this using the technique I decribed. If I had to do this day in and day out for a living, I would rig a fixture to hold the wood dowel. There are other ways to hold the front sight base, but I don’t want to see any new tool marks when I am finished with the job.
I have been told countless times that 13 clicks from mechanical windage zero is acceptable, to which I vehemently disagree. I have had to use this technique to zero windage on all but 3 of the AR carbines/rifles I have purchased since 1985 and have performed this task for several people I know in the shooting community.
