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View Full Version : Need advice on installing barrel and keeping FSB from canting



vingrjoe
8 October 2010, 13:46
I just recently assembled two seperate uppers. Both have Bushmaster 20" Govt. Profile barrels, one upper has a YHM A2 receiver and the other has a YHM A3 receiver. It took me about 69 ft lbs of torques on the A2 and about 62 ft lbs of torque on the A3 to get the barrel nut timed for the gas tube.

I took them to the range to zero the sights using the 25m targets found online. My A2 took 14 clicks to the left for windage and my A3 took 16 clicks to the left for windage. I am not happy with having that many clicks either way for windage. I suspect my FSB possibly were canted due to the torque being on the higher end. I used the Bushmaster clamshell receiver block when installing both barrels.

I was looking for some advice on this matter.

- Would lapping the end of the receivers where they mate up to the barrel collar help me to get the nut timed in at a lower torque ?

- Also, would lapping assist in reducing the amount of adjustment in windage and elevation to zero my rifles ?

- Is there any disadavantages to lapping my upper receiver ?

-Should I use barrel vise blocks like in the TM instead of the clamshel receiver block ?

-Or should I use barrel vise blocks in conjuction with the clamshell receiver block to secure the barrel when tightening ?

Thank you in advance, I look forward to any recommendations.

Quib
8 October 2010, 13:50
Post #5 in my AR FAQ Thread might have some helpful tips on FSB canting.

http://www.weaponevolution.com/forum/showthread.php?1613-FAQ-s-BASIC-INSPECTIONS-AND-PROCEDURES

vingrjoe
10 October 2010, 07:46
Thanks for the reply and link Quib.

I have read over that before, but I must admit I never followed those directions exactly. I honestly don't know what gets into me sometimes, just laziness maybe.

Would it be acceptable to clamp my barrel in a barrel vise once I have achieved aligniment between the bore and the iron sights ? This would be in addition to having my receiver in the clamshell block.

Thanks again.

Quib
10 October 2010, 08:07
Would it be acceptable to clamp my barrel in a barrel vise once I have achieved aligniment between the bore and the iron sights ? This would be in addition to having my receiver in the clamshell block.

Thanks again.


I don’t see any harm in that…..if you have two vices adjacent to each other, one holding the receiver in a clamshell style vice block, the other vice secured farther down the bench, holding the barrel in a set of barrel vice blocks.

For the sake of simplicity, I simply have a second person hold the upper receiver in alignment with a bronze drift through the FSB assy. This method has always given me good results.

Specialized Armament uses a similar method, with a tool they designed specifically for holding the FSB in position.

http://www.weaponevolution.com/forum/showthread.php?1971-AR-15-Tools

Quib
10 October 2010, 08:08
Another question that comes to mind….are you applying a sufficient amount of grease to all mating surfaces of the barrel, barrel nut and upper receiver? This “wet torque” will help tremendously in overcoming the drag encountered when tightening the barrel nut.



http://farm5.static.flickr.com/4103/5067558219_572cdc5a7e_b.jpg

http://farm5.static.flickr.com/4086/5068169088_908f5cbd6a_b.jpg

http://farm5.static.flickr.com/4149/5068168788_234d30d106_b.jpg