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Posted: 8/3/2008 6:42:27 AM EDT
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OK, I don't have the parts specs because it's a weekend and cannot call Bushmaster. One of my Bushmaster lowers with a RRA marked CAR M4 style stock will not allow the bolt to fully retract. The upper's will function fine on my other lower. I cannot retract the bolt enough to have the magazine catch engage. It is not a out of spec buttstock that gets in the way of the charging handle. It matters not if it's my 6.8spc upper or any of my 5.56 uppers. The spring is 10.75" long The buffer is 3.25" long I bought the stock assembly used and it appears to be normal and not an Airsoft knock off. Any suggestions? |
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Here is something else that you might want to check... Check action spring for kinks and free length Free length should be... RIFLE: 11 3/4 Inches (29 85 cm) minimum to 13 1/2 inches (34 29 cm) maximum CARBINE: 10 1/16 inches (25.56 cm) minimum to 11 1/4 inches (28.58 cm) maximum. Do not attempt to adjust spring length |
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The springs check ok for length, but they have tapered ends. I do not remember any other carbine springs having tapered ends. The stock tube has an indent on the back wall where the spring should fit, but I am not sure that it is. Any chance that the bolt catch is under too much spring pressure and it retards the bolts aft movement? Prior to this, I was having problems with this lower not locking the bolt back on an empty magazine. |
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Is the spring coil end on the higher ridge area of the buffer? If yes, pull the spring and try just the buffer without it. If you have a carbine receiver extension, and carbine buffer, and the bolt will not retract back past the bolt hold open device, them chances are the back void of the receiver extension was not milled far enough back. Also, the back of the tube cavity should not have a ring edge step, it should be square/flat. |
It has a step, the stock is RRA, I am not sure the tube is, it could be an aftermarket part or from an Airsoft, I bought the stock used. |
Sounds like you need a new carbine receiver extension. Just remember to order the OD size to fit the Butt stock in hand. |
I didn’t bother going there because according to the length he posted above, his spring falls within limits. 
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Yes it does, and the magazines lock in place just fine. I went ahead and ordered a new tube from RRA along with a buffer and spring and also spare springs for the bolt hold open latch. If that does not fix it, I'm not sure just replacing the whole unit in not in order. |
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Just for a comparison, I measured my RRA receiver extension interior, H-buffer and buffer spring overall lengths. Receiver Extension interior: 6-15/16” measured at the top of the tube, not down by the buffer detent. H-buffer: overall length 3-¼” Buffer Spring: overall length 11-1/16” |
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The very same thing happened with my Bushmaster. You have to see if the 2 hex bolts that hold that thing on top of the bolt carrier are tight, (I think is 20 pounds tork) the gas was escaping through the bottom of that piece. Mine were loose and after I made them tight, problem solved. |
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