AR Sponsor
Posted: 3/28/2007 2:23:22 PM EDT
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Need some help solving a case problem: AR-15 in 17 Mach IV has had two ruptured cases with normal loads, nothing hot or near max. I have a Tubbs counterweight in the bolt. When I cycled the gun manually the bolt tends to stick and you have very hard time pulling the bolt back. When I removed the weight, the gun cycles normally. My question is could the extra weight in the bolt be tearing up the brass in some way? And any ideas on why the extra weight makes the bolt stick? Any help would be appreciated. |
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When the spent case is extracted, it's still under barrel bore backpressure. With the extraction slowed down in movement, it is possible for the back of the case to be unsupported with the front of the case still sealed off (resealing), causing the back of the case to let go on the cycle. Granted that you will not get the full 50,000 DSI of blow out, something in the 20K range is enough to let you know that you do have problems with the case spit and blast pressure into the action when the retracting speed is slowed down. Also, the question comes into play in regards to the gunpowder being used. With slower powders, the unlock pressure will be higher since the gun powder pressure peak spike is closer to the gas port, and more of an issue in your case. Since you have not stated barrel length, gas port from muzzle, and again powder, you will have to take these into your own considerations/factors. And regards to the case sticking in manual mode, it's not uncommon since the bolt gets pinned to the rear/tight against the barrel extension as the pressure builds, and the slightly pushes the expanded case back into the chamber as the pressure resides/as the bolt un-cams off the extension. Combine that with a action unlike a bolt action where you direct cam the bolt out of the play, the tension of the charging handle pulling the front of the carrier back, which has to transfer to the cam against the upper bearing track (more that a few tension points), and spent case in the chamber may be a bit more difficult to extract. And, to go back to the burning rate of the gun powder, If the powders that are beyond too slow for the auto actions are used, there is soot'g of the case (fouling) causes by too much blow back around the case before it seals off to the chamber. Federal had this problem with a few lots of Gold in 223, and was so bad that it even lead to primer blowouts as well. Bottom line is you figured out that the rifle and the ammo you are reloading does not require, nor desire the additional weight that you have added. Also, in regards to the manual mode, just short of slead’g to insure non bullet tip deformation, you may find that going manual to self cycling if you are feeding out of the mag may not be the big difference in group size that you expect to find. Truth be told, If/with cases sized on the larger dimensions, there is a chance of the non consistent bolt lock up in the manual mode, and self cycling mode may be more accurate/consistent. |
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Thanks for the info, one correction on my part: When I mentioned I cycled the gun manually, I did not fire the round. I would pull the charging handle back all the way and let go. I did the same to remove the round, but could not pull the charging handle back. When I did the same drill without the CWS, I could eject the unfired round with normal pressure. The rifle seems to cycle fine when a round is fired. The rifle is an AR-15 in 17 Mach IV with a 24 inch barrel and I am using RL-10X powder. I was playing with it manually and not firing it when I discovered this issue. Any input would be appreciated. |
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With the only differences between the dry cycles being the Tubbs weight then not, I would venture that the tubes weight is somehow bind up the works (that is as long as you are full sizing correctly, and that you have not long loaded the bullet, nor buckled the shoulder by over crimping). As for the powder, I can't find a burn rate for it, but with the longer barrel and lighter bullet, you will be playing the fence on clean cycling with faster powders, to faster bullet speeds with a possibility of soot'g/ spent case over pressurization to the chamber wall at extraction/blown primers. I think if you start checking the loading data, you are going to find that most of the loads are tailored around the rig being a bolt action one, and when working up loads for the autoloader, you may wish to start off with the faster burning powders first to work up loads. Granted with the longer barrel, the slower burning powders are going to give you more speed, again, it’s the auto cycling action that was designed around a specific burn rate (powder spike peaks before the bullet gets to the barrel gas port) that you will have to weigh into the mix for the best overall load (speed and cycling). |
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