Bolt not locking - Buffer too short??!

Keebler750

Keebler750

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Originally, this was a post in Service Rifle about first round feeding, but I've figured that out while finding this NEW problem. I moved it here to the appropriate forum.

I just bought my first black rifle, and haven't even fired it yet....ATT!! :mad: :rolleyes: I have two complete stocks, springs and buffers from Bushmaster. The Non-adjustable came on the rifle, but I put the carbine stock on.

My bolt isn't going back far enough to lock properly, the bolt lock is catching on the carrier instead of the bolt face. I went looking for what was blocking it from going rearward....I've tracked it down to the buffer spring coil-binding (fully compressed) and not allowing the bolt to come back all the way with the charging handle.

I haven't fired it in this condition.

Only my carbine stock does this, no matter which spring I use. (I'm using the short recoil buffer btw.) I am still trying to figure out why this is happening but it appears the buffer tube is .250" too short. (or I've been given the wrong second spring.)

The non-adjustable stock, buffer and bolt lock is working fine.

I backed off the extension tube one turn and then it worked but the detent for the buffer blew out and jammed in my bolt and reciever, making disassembly "challenging." :)

I looked at an AR at the range on Sunday to confirm the bolt issue. Now I have to figure out why it won't go back far enough to lock properly.

DETAILS
The spring that was in the 'rifle' stock was longer. I put in the shorter spring.....it locked back no problem with this one as well.

The spring that was in the carbine stock was shorter. I put in the LONGER one and it worked too!!! It locks back.

Now I'm really confused. Why would a longer spring of the same diameter wire not bind? I should count the coils just 'cause I'm really bored.....

Silver spring - 12" long, 43 coils, 0.072" wire....achieves coil bind 1/16th " sooner than the Bronze spring which is - 12.75" long, 43 coils, 0.072" wire.

The Bronze spring in the carbine stock allows manual bolt lock RIGHT AT the point of coil bind therefore in actual shooting conditions I am doubting it will lock properly. You dont want the springs bottoming from the recoil, and you want SOME over-travel I figure.

The 'rifle' stock allows the bolt to over-travel the bolt lock (rearward) by 0.25". The carbine stock just BARELY lets the bolt lock.

I think my carbine buffer tube must be too short inside.....it is 7" long on the inside but 7.75" on the outside, meaning they plugged and welded the tube with a .75" spacer when they built it. In order to work the same as the 'rifle' stock it looks like it needs to be around 7.25" inside.

Anyone willing to measure their tubes and springs for comparison? Any ideas?

For now I'm going with the rifle stock until I figure this out.
 
You've got two rifle springs. Neither will work in a carbine receiver extension (aka buffer tube).

These numbers are from a Colt first pattern M4 4-position collapsible stock, c. 1999 or so:

receiver extension is 7-1/4" long
H-buffer is 3-1/4" long
Recoil spring is 10-5/8" long, 38 coils

Additionally, according to my reprint of USMC TM 05538C-23&P/2 (Oct. '84), any spring with a free length of between 11-3/4" and 13-1/2" is considered acceptable for use in the M16A2 rifle.

A lot of those fixed stocks that looked like collapsible stocks were built on rifle length extensions. This is why both your rifle springs work in them. But neither will work in a proper carbine length extension.
 
So, I'm wondering why Bushmaster shipped me springs or parts with this brand new rifle that don't work.

I noticed you said the reciever extension is 7-1/4"... is that the inside dimension? Because as you see above mine is 7" and I think that's the reason it's bottoming out.
 
Keebler750 said:
So, I'm wondering why Bushmaster shipped me springs or parts with this brand new rifle that don't work.

I noticed you said the reciever extension is 7-1/4"... is that the inside dimension? Because as you see above mine is 7" and I think that's the reason it's bottoming out.
Click to expand...

Nope. 7-1/4" is the external overall length.

Hey mistakes happen. If you don't want to send the whole thing back, a new carbine spring is usually less than $20. Or maybe you could drive up to William's Arms and compare yours to one they have in stock.
 
FWIW - you can cut a rifle spring -- I chopped a few C7 springs in my time to fit C8 buttstocks on. I wont guarantee the functioning - but its an option. The M4 buffer spring is supposed to be 10 1/16th" to 11 1/4"
Rifle is 11 3/4" to 13 1/2"
 
Thanks for the help guys. You were right.....Springs!!!

I just borrowed someone elses factory carbine buttstock to compare.

The tube dimensions are identical. It's obviously not that.

The SPRING in this one is......10.5 inches long and 38 coils. My shortest is 12.0" 43 coils. Five extra coils stacked at coil-bind is .072" x 5 = 0.360" !!!

There ya go! They shipped the wrong spring and they go into coil bind sooner, causing my bolt lock issue. Well, I was looking in the right place, anyhow!!

I have thought about chopping 5 coils but it will make the spring 13% stiffer (Shorter spring is proportionally stiffer. Inverse of 38/43...but I forget the darn math).

Will that be a problem for proper action?
 
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Are you using the right buffer in the right stock? You don't have a rifle buffer in the carbine stock by chance do you.......
 
I'd just email Wolverine, TSe or P&D for a new spring - they are cheap -- and bust out the person/place/thing that sold/shipped you the wrong one.

-- I've gone so far as to cut down a rifle buffer to work in a carbine system -- not as easy as it sounds (you need to cut down a carbine spring by 3 coils to make up for the decrease in allowable length on the chop rifle buffer due to its double shoulder)

Anything is possible if you take the time and effort -- but its easiest to simply order a new one.
 
Well. I've cut the coils and brought it down to spec, and the bolt goes back past the lock by .250 and then locks. Looks like I'm good to go. I even put a decent end to the last coil so it doesn't eat the aluminum stop over time.

I think I'll order a new one, too.

Three months from now, after I get my ATT, I let you know how it works :rolleyes:
 
Keebler750 said:
Thanks for the help guys. You were right.....Springs!!!

I have thought about chopping 5 coils but it will make the spring 13% stiffer (Shorter spring is proportionally stiffer. Inverse of 38/43...but I forget the darn math).

Will that be a problem for proper action?
Click to expand...

Thought so.;)

I am not a spring expert, and will happily defer to any person who is, but are you sure about your statement above? I just cannot see how the overall equilibrium length of a spring could affect its force constant. Now, if you were to compress it from 12" to 10.5", yes, that would up the force delivered by 13%. But I don't understand a change based simply on a difference in free length.

I bet a neatly-trimmed spring will work just fine.
 
I was monkeying around with a friend's new Bushmaster earlier this evening. It came in the wretched preban configuration that the DOS seems to only be approving export permits for currently. It had a fixed stock, pinned on muzzle brake and no bayonet lug. It also came with an extra ACE telescopic stock. I'm suspecting that some of the importers on our end are swapping the fixed stocks off for telescopic ones once they get across the border and some of them are occasionally gimping up putting the right parts back in.
 
Yes its true about cut springs being stiffer. Just ask the car guys. Actually it is suspension technology that I get this info from, but I'm not sure of the exact calculation anymore. If you're crazy (and bored), I'll explain.....

A coil spring is, in effect, a torsion bar neatly wrapped in a compact shape. As you compress the spring, the cross-section of the spring twists, and resists the compression. The shorter a torsion bar is, for the same material and cross-sectional diameter, the stiffer it will be when you hold one end and rotate the other. Imagine a long bar twisting, and how hard it would be to twist the same bar if you cut it to 25% of its original length.

If you shorten a coil spring, but don't modify it in any other way, you are removing the amount of material available for twisting about the cross-section.

It becomes stiffer by a mathematically proportional amount.

Some Spring Law......I forget which.

Also spring stiffness is not the same as spring pressure, or force. Stiffness is the rate increase of force stored in a spring per unit compression (like....100lbs per inch.....that's a stiffness rate you can use to compare springs.), while spring pressure (which is the force you feel when you compress a spring) is the amount of stored energy in a spring, put into the spring as a result of the applied force.

If you put a spring on a scale, zero it, and apply a downward force on the free end of 300lbs, the scale would read 300lbs and that amount would also be stored in the spring until the force was released. If the spring compressed 3 inches when you applied this 300lb force, you would have the above mentioned 100lb per inch spring :runaway:

Inquiring minds............ :D



(I'm an ex-motorcycle racer, crew chief, and research-addled fool.)


BattleRife said:
Thought so.;)

I am not a spring expert, and will happily defer to any person who is, but are you sure about your statement above? I just cannot see how the overall equilibrium length of a spring could affect its force constant. Now, if you were to compress it from 12" to 10.5", yes, that would up the force delivered by 13%. But I don't understand a change based simply on a difference in free length.

I bet a neatly-trimmed spring will work just fine.
Click to expand...
 
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