EricW Posted August 18, 2004 Share Posted August 18, 2004 I'm thinking about buying an extra bolt or two to leave in the buttstock as a quick spare in case I have a gas ring/ejection problem during a match. I *think* I remember Dr. Sweeney saying in the past that changing bolts can change the headspacing on a rifle. Can someone clue me in on just how much variance I can expect if this is indeed the case? How the heck do you check? Not that I won't check this myself when I get the bolt(s), but can changing bolts substantially affect a rifles zero? (more than fractions of a MOA) [Thanks for answering all my goofy rifle questions] Link to comment Share on other sites More sharing options...
Pact-Man Posted August 18, 2004 Share Posted August 18, 2004 Eric, AR bolts are all held to tight enough tolerances that they are interchangeable (for the most part) when assembling a new bolt/barrel/extension. However, as the parts "settle in" with each other, there will be a certain amount of "play" that develops in the combination. Not enough to cause excess headspace, but maybe .001" or .002" extra compared to a new assembly. Headspace "range" is in the neighborhood of .005" from a typical GO dimension to a typical NO GO dimension. What does this mean? Chances are good that you could swap a used bolt for a new one and be safe and functional. However, there will be differences in the mating surfaces, and this COULD cause problems with functioning or accuracy. It is not something I would do if I had a choice, but I am sure people do it when they need to. Just be sure to check headspace with both bolts and function test them before putting one aside. Also, try to stick with the same manufacturer for all the bolts. Link to comment Share on other sites More sharing options...
Patrick Sweeney Posted August 18, 2004 Share Posted August 18, 2004 Funny you should ask, I was just going over this in the latest book. And the answer is a qualified....Maybe. Yes, mil-spec bolts in mil-spec barrels Should interchange without a problem. But the military standards are not what we would feel comfortable with in a match. Under certain circumstances, a military armorer might rebuild a rifle using a used bolt and barrel, the combination of which would gobble a "NO-GO" gauge without a hesitation. Who cares about case stretch? (As long as it doesn't accept the Field gauge, get back out there and shoot more bad guys.) The problem comes in that not all bolts or barrels are mil-spec. I've seen lots of "mil-spec" parts that were grossly out of headspace when put together. 'Way under, and the bolt won't even close on a cartridge. Over, and brass has a useful life of one firing. Zero shifts can easily happen. Again, in military use, who gives a flying fart if rebuilding a rifle causes it to change by "minute of jihadi" you can always re-zero. (You can even re-zero in the middle of a fight, as my Dad did one afternoon in Germany.) But a 1-MOA zero shift in an unknown direction in a match can be a problem. And 2 or 3 MOA is not unusual. The answer for you EricW my friend, comes from Ronald Reagan: "Trust, but verify." If you simply must have a spare bolt, get headspace gauges. Get a chambering reamer (finish, 5.56, not .223) if need be and use a carbon steel or stainless barrel. With luck, you can get the headspace reamed to accept the GO gauge for both bolts, and not accept the NO-GO for either. The best solution is to have a gunsmith with a bin of bolts on hand do it, so he can measure the bolts until he finds a matched pair, then headspace. To answer the basic question, do you need a spare bolt? Probably not. What malfunction of a bolt could happen, that wouldn't trash the rifle, that you could fix by the next stage? Spare extractor, sure. But bolt? I travel with a truckload of gear, and I don't carry a spare bolt to a 3-gun match. Link to comment Share on other sites More sharing options...
KyleL Posted August 18, 2004 Share Posted August 18, 2004 Eric, Patrick is right on the money, it is easy enough to check the parts before a match. However, I have only seen a couple of bolts break, two of those were in Prototype AR's that were putting too much pressure on the bolt lugs. I reccomend you carry a few gas rings, extractor, extractor spring, spring bumper. If you do buy AR spare parts, try to avoid the lowest bidder, get some quality back up parts. God Bless the USA!!!! KyleL Link to comment Share on other sites More sharing options...
EricW Posted August 18, 2004 Author Share Posted August 18, 2004 Thanks guys. I didn't want an extra bolt because I thought the bolt would break. I just thought it would be easier than trying to install a new extractor / spring / gas rings in the field. Sounds like the logistics of headspacing the bolt vastly outweigh the inconvenience. A baggie of spare parts it is! Link to comment Share on other sites More sharing options...
BPiatt Posted August 22, 2004 Share Posted August 22, 2004 You can also compare two bolts using simple micrometer. If you can pick from a few bolts, find one with the same dimensions as yours. I've only found 1 or 2 thousanths between the 5 or 6 bolts I've measured. 1. measure the length of the locking lugs. 2. measure the depth of the bolt face. 3. subtract the numbers and you have a number you can compare with. Remember, the bolt locks on the back of the lugs. Bruce Bruce. Link to comment Share on other sites More sharing options...
Paule Posted August 31, 2004 Share Posted August 31, 2004 All bolt makers are not equal. We used to scramble whenever we needed two bolts headspaced to a single barrel extension. A sackful of bolts was mandatory. Switched bolt suppliers a couple years ago and now it's almost automatic. Virtually impossible to tell the difference with GO-NOGO gauges. I can see .0001-.0003" variances with a micrometer that reads to 1/10,000th. We still check every one though. (I have plenty of belts AND suspenders too!) Seriously, better to know it's safe... Most bolt failures we have seen were in f/a use, usually after 25-30k rounds. Lugs cracking is the most common occurance. Link to comment Share on other sites More sharing options...
kurtm Posted September 1, 2004 Share Posted September 1, 2004 I have never seen head space effect accuracy much. I once tried a simple test with a Remington 700. It was a 1 MOA rifle that a guy wanted rebarreled. I took the barrel off and reamed it .005 at a time, reinstalled and tested untill it was WAY past a field gauge. The brass suffered, the primers fell out, but it would always hold about 1 MOA. I did notice some vetical stringing, and the velocity kept climbing as the head space "grew". the point of impact shifted about 1/2" at 100 vetically until the head space was .02 past field! then it came right back to the zero it previously had, this may have been due to installing and uninstalling the action in the stock even though I was using a torque wrench to try to make sure all was the same. Malfunctions...maybe, blown brass...possiblly. Big impact shifts?? I don't think so. If you are with in a few thou. you should be OK! Only a Bench Rester would notice the difference in groups! I finally reached a point where everything remained the same, as long as the cartridge was "snapped" into the extractor by hand, otherwize it just wouldn't fire, the firing pin just wouldn't reach the cartridge. KURTM Link to comment Share on other sites More sharing options...
EricW Posted September 1, 2004 Author Share Posted September 1, 2004 Switched bolt suppliers a couple years ago and now it's almost automatic. Virtually impossible to tell the difference with GO-NOGO gauges. Care to share what manufacturer the good bolts come from? Link to comment Share on other sites More sharing options...
Paule Posted September 1, 2004 Share Posted September 1, 2004 The bolts we use come from LMT. (Lewis Machine & Tool) Link to comment Share on other sites More sharing options...
diehli Posted September 2, 2004 Share Posted September 2, 2004 Info on the LMT bolt: http://www.globaltactical.com/axami/shop.php?grd=128&prd=132 Link to comment Share on other sites More sharing options...
EricW Posted September 2, 2004 Author Share Posted September 2, 2004 OK, now I'm confused. Does LMT make normal AR-15 / M-16 bolts, or only the "enhanced" bolt. I have run one of these bolts in my M16 over 1,200 round with no cleaning, had 0 malfuctions due to failures to feed or extract. If you want absolutely the best bolt available today the LMT enhanced is it. It is not cheap but using the best never leads to disappointment. Groan... I've run a lot more than 1200 rounds with my stock bolt and had zero failures to extract too. And I can buy THREE, count 'em THREE DPMS bolts for the price of a bolt that fixes a problem that doesn't exist. Keep your foo foo parts. Give me Mil-Spec parts that are actually to print...and work. Link to comment Share on other sites More sharing options...
Paule Posted September 2, 2004 Share Posted September 2, 2004 My comment was about standard bolts, not the LMT enhanced bolts. Those were developed to meet a spec the military wanted. Several companies built enhanced bolts to attempt to meet the requirement. The most well known are the LMT and KAC offerings. They were not designed as "boutique" parts, the features included were the response to requests for longer life and higher reliability under some specific severe use conditions. Definitely not for everyone... Link to comment Share on other sites More sharing options...
Lycanthrope Posted September 6, 2004 Share Posted September 6, 2004 Patrick Sweeney nailed it. Any other info can be found on www.ar15.com Link to comment Share on other sites More sharing options...
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