Alan Waters Posted November 10, 2018 Report Share Posted November 10, 2018 Front of bolt carrier contacts rear of barrel extension. Pushes barrel extension forward .025. How much clearance needed between the two? Quote Link to comment Share on other sites More sharing options...
98Z5V Posted November 10, 2018 Report Share Posted November 10, 2018 You sure the bolt is fully locked, and back in the BCG body when this happens? Can you push the barrel extension back in there and seat it, without moving the BCG body back in the upper? Better way to test it would be lock that extension onto the upper with a barrel and barrel nut. Try to seat the BCG fully. Quote Link to comment Share on other sites More sharing options...
Matt.Cross Posted November 10, 2018 Report Share Posted November 10, 2018 14 minutes ago, 98Z5V said: You sure the bolt is fully locked, and back in the BCG body when this happens? Can you push the barrel extension back in there and seat it, without moving the BCG body back in the upper? Better way to test it would be lock that extension onto the upper with a barrel and barrel nut. Try to seat the BCG fully. He doesn't have a barrel nut at this point, that and a FFHG are in the works. To your question, I don't know that there definitively has to be clearance between the barrel extension and the BCG. The end result will be where the BCG will stop when the breach face sets the bolt to the fully rearward "locked" position. Impacting the barrel extension with the BCG is unquestionably a bad idea. Ideally I would want to know the distance from the front of the BCG to the face of the bolt locking lugs when the bolt is fully locked, then use that to help determine the relationship between the breach face and the locking shoulders on the barrel extension. Quote Link to comment Share on other sites More sharing options...
98Z5V Posted November 10, 2018 Report Share Posted November 10, 2018 We need @308kiwi in this one - he will have an idea, for certain. Quote Link to comment Share on other sites More sharing options...
Alan Waters Posted November 10, 2018 Author Report Share Posted November 10, 2018 1 hour ago, 98Z5V said: You sure the bolt is fully locked, and back in the BCG body when this happens? Can you push the barrel extension back in there and seat it, without moving the BCG body back in the upper? Better way to test it would be lock that extension onto the upper with a barrel and barrel nut. Try to seat the BCG fully. Yes. Too both questions. Pushing on the extension will push the carrier to the rear and the extension will seat properly. I will remove the bolt from the carrier in the morning and have a closer look. May have to face the extension about .030. Quote Link to comment Share on other sites More sharing options...
98Z5V Posted November 10, 2018 Report Share Posted November 10, 2018 Is this a raw extension? You're building the barrel, step by step, correct? Quote Link to comment Share on other sites More sharing options...
308kiwi Posted November 10, 2018 Report Share Posted November 10, 2018 Not sure if I am reading the OP's question/post correctly BUT................ The front face of the carrier stops against the rear face of the barrel extension, this is how it is designed to work, the bolt stops against the breech face, (rear face of the barrel), the carrier keeps moving forward camming the bolt around to lock via the cam pin and then stops against the rear face of the barrel extension. When the bolt is locked into the barrel extension there should be no clearance between the front of the carrier and the rear of the barrel extension. If you have clearance between the carrier and the barrel extension then you have a problem, in that the cam pin is now acting as the 'stop' mechanism for the entire BCG and all it's momentum when it stops in battery. The cam track in the carrier is ever so slightly longer that the length of the locking lugs in the barrel extension, this means the cam pin reaches it's most rearward travel, (when the bolt if fully cammed and locked), without reaching the end of the cam track because the carrier has come to rest against the back face of the barrel extension and the bolt lugs are sitting neatly between the front face of the extension lugs and the breechface. Have a look at any AR system that has plenty of rounds through it, you'll see a good solid witness of the front of the carrier on the rear of the barrel extension, a little extra proof that this is how it works is the phenomenon of bolt bounce on full auto M16's, carrier hits the extension with so much momentum that it bounces back, slightly unlocking the BCG, BUT on full auto the hammer has been released just before the BCG goes fully into battery, the BCG bouncing back off the extension meets the hammer as it's falling, BCG partly unlocked, firing pin is now not long enough to reach the primer, hammer hits the bottom edge of the BCG and the whole lot rides into battery, resulting in the hammer down on a live round. The only way to check what you are asking Alan Waters is to do so with an extension that has been properly fitted to a barrel and the barrel is located and secured into the upper, from the way the OP's posts read this is not what he is doing and he's seeing an issue that will go away when the rifle is assembled, (correctly), and as a whole. Some pics would help Alan Waters, trying to guess context leads to all sorts of confusion? Quote Link to comment Share on other sites More sharing options...
Alan Waters Posted November 10, 2018 Author Report Share Posted November 10, 2018 10 hours ago, 98Z5V said: Is this a raw extension? You're building the barrel, step by step, correct? Yes. Quote Link to comment Share on other sites More sharing options...
Alan Waters Posted November 10, 2018 Author Report Share Posted November 10, 2018 Thank you Kiwi308. Understood. Quote Link to comment Share on other sites More sharing options...
308kiwi Posted November 10, 2018 Report Share Posted November 10, 2018 What brand of barrel extension are you using??, I've only ever used BAT Machine 308 extensions and they are to spec, (if there is such a thing), point is you will not need to modify them. Even amongst the plethora of AR15 extension manufacturers I've fitted, damn I'd couldn't even tell ya how many, barrels/extensions, to the small frame guns and have never had to alter the extension specs, and even if you did try, I'd say damn good luck to you, they are as hard as the hobs of hell, you'll need some pretty decent tooling to do so. Quote Link to comment Share on other sites More sharing options...
Alan Waters Posted November 11, 2018 Author Report Share Posted November 11, 2018 Barrel extension is made by Dan Lilja. And yes they are hard. A file just skates over it. Not going to cut it. Quote Link to comment Share on other sites More sharing options...
survivalshop Posted November 11, 2018 Report Share Posted November 11, 2018 Is there a difference in Barrel extensions for an Armalite AR 10 & a DPMS LR 308 ? Quote Link to comment Share on other sites More sharing options...
98Z5V Posted November 11, 2018 Report Share Posted November 11, 2018 8 minutes ago, survivalshop said: Is there a difference in Barrel extensions for an Armalite AR 10 & a DPMS LR 308 ? You know there is - we've already seen the differences in the bolts, and BCGs... Quote Link to comment Share on other sites More sharing options...
Alan Waters Posted November 12, 2018 Author Report Share Posted November 12, 2018 10. Front/Back Bolt Play: If accuracy is the game, don’t leave a lot of front/back bolt play (keep it .003″ but no more than .005″). We’ve seen factory rifles run .012″ to .015″ play, which is OK if you need to leave room for dirt and grime in a military application. However, that amount of play is not ideal for a high-accuracy AR build. A lot of front/back bolt play allows rounds to be hammered into the chamber and actually re-formed in a non-consistent way, as they are loaded into the chamber. Excerpt from 6mm BR .COM ARTICLE ON THE ACCURATE AR Quote Link to comment Share on other sites More sharing options...
308kiwi Posted November 12, 2018 Report Share Posted November 12, 2018 (edited) Agreed, you only need enough clearance to allow the bolt to rotate and lock, .002" - .003", any excess and the bolt lugs will end up getting hammered into the extension lugs, hard on the lugs, hard on brass, (case rims) and extractors. The last large frame AR I barrelled, in 243 Win, I set the bolt clearance @ .0005", you can almost feel the lugs moving across each other if you manually insert the bolt and rotate it in the extension ?, but my theory being it gets cleaned enough that powder residue etc will not be a problem. It's been 100% reliable and has close to 600 rnds, suppressed, through it now. Edited November 12, 2018 by 308kiwi Quote Link to comment Share on other sites More sharing options...
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