The length of the proper buffer spring is a function of normal stock vs telestock right or is it barrel length?

Thanks
M4-AK

It has nothing to do with barrel length.

[img]http://www.imagestation.com/picture/sraid66/p4f713e0b2fe99496c1bde543142de8e8/fbe2941e.jpg[/img]

The receiver extension length dictates the buffer and spring length used in the rifle.  As the buffer moves back into the receiver extension, the spring comes just short of fully compressing.  If you were to use a full-length spring in a CAR rifle, the spring would bind up before the buffer made it all the way back.

Also, the length of the buffer determines the length that the carrier can move back in the upper.  If you were to use a Car buffer in a standard stock rifle, the buffer would be too short and the carrier key would make contact with the back of the upper receiver/cocking handle and crack the back off the upper as the rifle was fired.

Should the receiver extension be cleaned and or lubricated occasionally? Any problems to look for in that area that could cause short stroking?

M4-AK

You can go either way and shouldn’t have problems, as long as you don't over lube and attract crap in the extension.

If you think that the buffer/spring is causing you problems, just pull them and check them out.  You might have to clean up the edges on the spring is you are getting scraping marks.  Just use a file to clean up the end cut if you have a bur sticking out and doing damage to the tube.  
On the buffer, you may want to straight file the buffer roll pin is it's sticking out and catching the spring when the buffer comes back.


The one thing that I should add is if you have having problems with the rifle, and your using a new stronger recoil spring (like a extra power), you may want to just find a old spring until the rifle breaks in, then drop in the stronger spring.  
Clipping a few winds off the spring should never be an option, nor desired.

P.S. I'm in Palm Beach until the end of the month. If you’re close, and what to get together for me to look at the rifle, drop me an I.M. I could use the time off from golfing and get some trigger time in, instead of spending my weekends at local courses sweating my ass off.

Re-uppered a 14.5 barrel with a new flat-top upper. Lower previously used on a 20 is broken in thouroghly. Getting some failures to lock back. Shot enough rounds (200+) over various sessions that would have thought the bolt carrier would have mated by now but still doesn't consistently lock back.  

In comparison with others though, was definitely not a loose fit. Bolt carrier was not binding in the upper, no rough spots just seemed almost like a piston in a cylinder.
 
Finally took some fine sand paper and lightly sanded the rail anodizing in the upper.
It appears to be as loose as others now.  Didn't check it before sanding but Empty BC will slide out if barrel is tipped now.

I don't believe gas tube was binding and no gas leaks found either.


Haven't shot it since sanding. Figured that will solve it. I suspect that drag from close tolerance was causing short stroking.

Have noticed when the charging handle is pulled fully rearward that the spring seems tight when nears full compression compared to other lowers. Almost as if something is tighter in there. I want to check it out.

Will check my buffer and spring today. I Have a replacment Bushie spring but no new buffer. Will see if spring is rough or scraping and or if buffer pin is rubbing as you suggest.

As for geographics. I'm near Crestview near alabama. Not close to Palm Beach. But thanks for the offer.

M4-AK

Not the best way to solve the problem.  The simpler way is to just check the four bearing corners of the carrier key and remove any sharp edges that may be grabbing the upper slot.  The upper key slot keeps the carrier in line, and if you remove too much metal, it allows the carrier to twist too much and prevents the bolt from making a clean disengagement from the barrel extension.  As stated, the problem is one of the key corners grabbing the upper and slowing down the carrier travel.




You may want to check the hammer to see is it is binding up on the disconnector.  Look for a white line/dent just behind the hook.  Granted that the old upper worked well with the lower, then new upper may still be tight and keeping the carrier lower against the hammer.  If you have signs of the hammer striking the disconnector, then remove .040 off the hammer tail. This will allow the hammer to be lowered father down, without binding up the carrier.

Dano:

I had put about 300 rounds on this new upper combination 14.5 flat-top but it never seemed to loosen up.

I only made a couple of light strokes on the rails with superfine sandpaper last week. The drag now seems normal. Don't think I really took anything material off. Nothing visible on the anodize.

I'm thinking the problem was the bc slightly dragging in the upper, which it isn't now. Failure to lock was about 1/4 to 1/2 the time after it got hot.

Today I took the buffer and spring out.  I didn't feel the spring was catching but I filed the roll pin on one side of the buffer. It wasn't perfectly flush, and was slightly out  on one side. I inspected the spring and found the correct length 13" approx.

Looking down inside the extension with a light I could see a slight scratch or wear mark all the way down one side. Nothing you could feel. Also I can see that it's almost imperceptibly out of round at the very bottom where the grommet attaches?  

I lightly wiped the buffer, spring and extension and reassembled.

One other thing I've noticed that is different on this lower (A bushmaster put together by southeastern gun parts in Mobile,AL using their parts)from other 100% bushmasters is that pulling the charging handle back is smooth but that the last inch where the parts starts to bottom out is more draggy than other bushmasters.

Haven't had range time yet to see if any of this cured it.

If it isn't cured, any thoughts?

M4-AK

Re-check the gas system, you may have a leak between the key/carrier, even if the key screws are tight.   The fact that it's acting up when hot means your getting a gas leak, and not problems with the buffer/extension.

Think lapping the key to carrier, and cleaning up any burs that may be holding the two  slightly apart.   Also, a light coat of blue loctite between the key/carrier will seal off any voids that you don't lap out.