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Posted: 9/26/2009 5:51:42 PM EDT
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The outcome of the day's gun doings would have seemed unimaginable not long ago.
Recently I put together a carbine upper that was going to be so close to a Model 653 I decided to go ahead and make it somewhat of a retro: 
It awaits parts for its lower, but I had another AR whose upper had to go back to the manufacturer, so I decided to pop the 653 upper on the idle lower just to try it out. Zeroing proved impossible, as the A1 sight was still just hugging the right side of the bullseye with windage cranked to the max. Frustrating. While the barrel is brand new, the receiver is an old M16A1; thus I'd lapped the receiver face before assembly: 
Having already addressed that, I looked elsewhere for the problem. The barrel's a Bushie and I know they've a history of occasionally putting out barrels with canted FSBs. Visual inspection didn't reveal such, but that's not definitive. One fix some have suggested for that is a few good whacks with a rubber mallet, but that seems a last resort. I did consider that the old receiver might be warped, but a Rube Goldberg test failed to discern significant warpage: 
So, before I think about some serious mallet time I decided to try the barrel in another receiver, as I had a BCM flattop and an unused Magpul MBUS waiting on some other stuff. Thus tonight I've built a One Day Gun: 
The sole purpose of this assemblage will be to fire one 20 round magazine at the range tomorrow. After that the barrel, receiver, sight and lower will go their respective ways. I probably am doing this the hard way. Any alternate ideas are welcome. |
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A better idea perhaps. |
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Hmmm..., well, it zeroed, but only with the MBUS cranked one notch away from all the way over to one side. I put a red dot on it and it does put 'em in the same place, at least to the degree that I'm doing my part.
If it is misindexed, as I understand that, then the indexing pin and the gas port would be misaligned. With the FSB installed in the correct relationship to the gas port, wouldn't the result be a canted FSB? I'm not sure what you'd do to fix that. |
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Quoted:
Hmmm..., well, it zeroed, but only with the MBUS cranked one notch away from all the way over to one side. I put a red dot on it and it does put 'em in the same place, at least to the degree that I'm doing my part. If it is misindexed, as I understand that, then the indexing pin and the gas port would be misaligned. With the FSB installed in the correct relationship to the gas port, wouldn't the result be a canted FSB? I'm not sure what you'd do to fix that. ? There should not be the same impact shift with an optic mounted on the upper unless the upper IS the problem. Bill |
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It's fixed. The 653 upper's been reassembled and mated to its permanent lower. This morning it zeroed - still carnked over quite a bit, but consistently on the bullseye with some adjustment left.
Ultimately I guess the fix really amounted to taking it all apart and putting it back together again. |
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