I have a bushmaster m4 carbon 15.  I since I first got it, it shoots about 7" groups at 100.  I have tried different scopes with all the same results.  After giving the gun a good look over, I found that a screw holding in place the picatinny rail to the upper was stripped, and the scope could wobble slightly left to right on the base.

I re-tapped the screw to a bigger size, and put some JB weld under the rail to keep it more stable on the receiver.

The scope doesn't really noticeably wobble anymore (it might still barely, but not like before), but it still shoots like crap.  I've tried 55 and 60 grain ammo, with no difference.  The barrel feels sturdy, but would need a barrel wrench to know for sure.  I've also checked the crown with a q-tip, and seems fine.

What would you guys do?  Is it probably still the upper receiver base giving me trouble or should I start looking into other things?

Sounds like it's time to invest in a good mount.
Did you say you put JB weld between the rail and the mount?

Yeah, I put a little JB weld on to help minimize the wobbling side to side of the mount.

Can I get a new mount that will go on the upper receiver that will screw in and fit better than the stock one?  I thought on most ARs the scope mount/rail was forged as part of the receiver, not a removable part?  I think because the upper is carbon fiber, they had to attach a rail to the upper, instead of having in part of the upper.

Can you post some pics?  The Carbon Fiber one I had the top mount and the upper were all in one piece.  Mine shot easily 1in groups at a hundred with the ammo you mentioned.

here are the pics.  you should be able to see that the rail on the receiver is made of different material, not carbon fiber.  It screws into the receiver.  The original screw was completely stripped, so I tapped it and put in a bigger hex screw.

If you take the screw out, take off the forend/barrel, you can slide the rail off from the front.  If I took it off, could I still mount my scope the receiver, or do I need some kind rail?


http://i492.photobucket.com/albums/rr286/dejongbd/untitled1.jpg

http://i492.photobucket.com/albums/rr286/dejongbd/u2.jpg

To anyone who is interested, I confirmed that my Carbon 15 upper receiver was to blame for the accuracy problems.  I tried different bullets, scopes, bases, a free float hand guard, and putting jb weld between the picatiiny rail and the upper.  Even after that, I could grab the barrel, and grab the scope with separate hands, and torque them in different directions fairly easily.

To fix the problem I bought a new solid aluminum upper receiver, and kept all my old parts (barrel, scope, mounts, hand guard, ect.).  Right after I swapped out uppers I noticed I couldn't torque the scope and barrel in opposite directions.  I took the gun to the the range and it went from a 7 MOA to a 1 MOA (8 shot group).  For a Bushmaster M4, this is more like it!  To top it off, I was only using cheap re-manufactured ammo.

The new upper doesn't match with the Carbon 15 lower perfectly, but most people won't notice unless I point it out.  Nonetheless, problem solved!

If anyone else it having major accuracy problems with their Carbon 15, grab the barrel and scope to see if they torque in opposite directions.  Even a slight noticeable move can equal several inches at a 100 yards.