broken bolt in colt SP1....due to milsurp 5.56??

[carlrodd]

i borrowed a neighbour's SP1 and took it to the range two weeks ago. the ammunition i fired through it was surplus 5.56. i've read a couple of articles about civilian firearms not being able to handle the higher pressures of surplus ammunition, but given that he gave the ammunition to me, i just didn't think about it...i figured he had used it before with no problems. nonetheless, my range time with the rifle lasted as long as it took to fire five rounds. on the fifth round, one of teeth on the bolt face broke, and an unspent round was lodged in the chamber. i had to rod the round out, and obviously the rifle no longer functioned with it's broken bolt.

this SP1 was manufactured in the early 80s, so at first i thought it might have been a product of wear, but then i remembered the issue of extreme pressures from milsurp ammo. assuming that the bolt was not excessively worn, could five rounds of milsurp have done this to the bolt?

 

[TimboKhan]

I am sure more competent people than me will answer this question better, but my guess is that the mil-surp ammo didn't cause this.

 

[Bartholomew Roberts]

It happens to AR bolts from time to time and generally ammo is not the problem. The two most common causes I have seen or heard of are:

1) Hydrogen embrittlement from chromed bolts. Chroming can weaken the underlying steel and an AR bolt already has some major stress on the lugs. I've seen three DPMS chromed bolts (all from a 2000-2001 timeframe, so the problem may be corrected now) - two that broke lugs and one that snapped at the cam pin hole.

2) Corrosion at the base of the lug - if even minor rust or pitting forms at the base of the lugs (particularly on the lugs on either side of the extractor that are already undercut), it can seriously weaken the lug and allow cracks and/or breakage.

I haven't seen #2 personally yet; but it has been discussed in the NDIA presentations on the SOPMOD program and I think there are some maintenance bulletins out on the issue as well. Both of these issues are worse in carbine length gas systems where the stresses on the lugs are increased 150% over the 20" gas system.

Since an SP1 is designed to fire higher pressure military 5.56mm ammo, I doubt that the ammo had anything to do with it. My guess (and this is just a complete WAG without looking at the rifle) is that as an older rifle, there was some pitting at the base of the lugs and that over time a micro-crack developed and you just happened to draw the winning ticket on when that crack became a break.

 

[carlrodd]

bart, thanks for the info. is my research on pricing for replacement bolts accurate?....around $50?

 

[Bartholomew Roberts]

Depends on what type of bolt you plan to replace it with. The Colt bolts have a significantly more stringent QC process and are a lot more expensive (around $160 through Specialized Armament Warehouse). These bolts are shot-peened, proof-tested and magnafluxed (all to make things like this less likely). They will be marked C MP on the bolt usually. This would be the equivalent of the original SP1 bolt (although it would be brand new instead of 26 years old with however many rounds were on it).

You can also buy a normal AR bolt that will do just fine for about $60 from any of the major AR dealers (RRA, Bushmaster, DPMS, CMT/Stag, etc.). It won't have any of those features; but I've run many of those bolts plenty hard with no troubles. You may even be able to find one for $50; though generally I can't find one that cheap unless I shop Model 1, M&A or some comparable lower end AR dealer.

You might check CDNN. They were selling some surplus Colt M16A1 uppers that they had converted to SP1-style uppers. They also had Colt bolts with them IIRC. They might be able to get you a Colt bolt at a better price; but I couldn't say with any certainty as they have been sold out of the Colt uppers for awhile now.

I'd also add that as long as you remove the bits of broken lug (which can work their way into different places and cause function issues), the rifle will probably run with a lug missing. I've read of bolts with two lugs missing continuing to run and function even in full-auto M16s. However, I would agree that it is probably a bad idea to test your luck that way. This is more of a "Just in case you have to" type of situation.

 

[carlrodd]

thanks again for the good info sir.