Stlhead Posted December 14, 2013 Share Posted December 14, 2013 When you put the gas block back on is it firmly seated against the shoulder? Link to comment Share on other sites More sharing options...
Thehotrodpig Posted December 14, 2013 Author Share Posted December 14, 2013 When you put the gas block back on is it firmly seated against the shoulder? No, if had done so the holes would have been off by approximately .020. I measured the two holes from the shoulder and from the end of the block and placed the block accordingly. Link to comment Share on other sites More sharing options...
Thehotrodpig Posted December 14, 2013 Author Share Posted December 14, 2013 Thank you Tom, you have been very helpful. I have learned a lot with this build. I now have the buffer weight at 2.9 oz, just under what the jp buffer is at 3oz. I assured the gas is aligned and clear and I cleaned and lubed everything. I also confirmed the gas block is open and lock tighted there. I'm going to function test it in a few minutes. Link to comment Share on other sites More sharing options...
Thehotrodpig Posted December 14, 2013 Author Share Posted December 14, 2013 Just went down to my range and ran 10 or so 2 round mags through it. Combo of old worn out GI mags and new pmags. The gun picked up every round and locked back on every empty mag. I know it wasn't a thorough test session but it was promising. My neighbors don't complain about the pistols but when I break out the ar for too long they start to complain a bit. This is with full gas locktighted in place. The receiver and bcg are new and when I cleaned them after last testing they were pretty sticky. I think that my issues could be related to a stacking tolerances type deal. New receiver, bolt, bcg, gas port perhaps on the small side, perhaps I didn't get the gas system perfectly aligned or blown out. I also did not have the gas adjuster locktighted in place before. I also might have found the sweet spot on the buffer weight at 2.9oz. I don't know for sure but I'll continue to test and report back. Thanks again for all the help you guys have been. I appreciate it. Link to comment Share on other sites More sharing options...
openclassterror Posted December 15, 2013 Share Posted December 15, 2013 That does sound promising! When I assemble a new build, I use a thin liquid lube on the carrier to receiver interface (rails), almost dripping wet. I also use shooters choice red gun grease on the cam pin, and cycle the bolt head up and down the cam track about 20 times to distribute it in the slot. Even with high quality parts it sometimes takes a little running to wear all the parts in together. One thing to keep in mind is that we are taking a bunch of mismatched finely tuned parts and expecting them to run with the reliability of an overgassed, minute of barn carbine. Sometimes it takes a little tuning. I have learned way more about how these things run from troubleshooting match guns than in 10 yrs of general gunsmithing repairs. You have to admit, you have a lot more knowledge about how the components interact now than if it had just run out of the box, eh? Tom Link to comment Share on other sites More sharing options...
Thehotrodpig Posted December 15, 2013 Author Share Posted December 15, 2013 Yes I do Tom, thanks to you and others that have walked me through the tuning process. I have assembled several cookie cutter ar's and never had an issue. Like you said they were basically just like Legos and they worked fine but none shot as flat or smooth as this gun. I enjoy the tuning and understanding why and how parts interact with other parts. I am not done though. Time to play with the gas a bit more and mess with my muzzle brake to see if I can get it to shoot even flatter. I will update is I get further. Thanks again. PS, I still would be interested in replacing the spring. Tubbs, jp or? Link to comment Share on other sites More sharing options...
TitoR Posted December 15, 2013 Share Posted December 15, 2013 I had he same situation a year ago when I did my build. I essentially have the same setup. Carbon Arms 18" rifle gas barrel, JP adj gas block, JP Low mass BC, and low mass rifle buffer. With JP buffer spring it would not lock back with Tulammo, and was barely locking back with brass 55g factory ammo, with the GB fully open. With a standard rifle buffer spring, is currently working fine with brass ammo, but still can't cycle cheap steel case ammo. My conclusion was that the system needed break in and more gas, but the barrel gas port is optimized for non-adjustable GB, so I am waiting to get to the 1000 round mark before I do any other changes since the rifle is running very well. If you ever decide to drill out the gas port, please post your results. Link to comment Share on other sites More sharing options...
openclassterror Posted December 16, 2013 Share Posted December 16, 2013 General consensus is that JP captured spring can be fussy (though I have had no problems personally), and special rate springs are hard to come by. I have no experience with Tubb springs, though I have had universally good experiences with his bolt gun parts. Wolff also makes good quality AR springs. All 3 are generally known for good customer service and standing behind their products, so can't really go too wrong here, IMO Link to comment Share on other sites More sharing options...
Thehotrodpig Posted December 31, 2013 Author Share Posted December 31, 2013 Update. I have a couple hundred rounds through the gun now and it has run flawlessly. Shot it back to back with a buddy's Rainier Arms 16" mid gas ar and there was a noticeable difference in how flat shooting my gun was over his. That is with my gas all the way open too. Shot it Saturday at 100 yards to see what groups I could get out of this new Criterion barrel. With the large dot of my Tac30 scope I was still able to shoot several sub 1" groups with my cheap Remington green box ammo. I think with a more target friendly reticule this gun has some serious potential. That Criterion barrel seems to be a good value. Overall for my first dedicated 3 gun build I think I have a very functional budget gun rifle. I want to thank all of you (especially you Tom) that helped me work through the build and sort out the gas issues. Now I need to just finish fine tuning the comp and possibly pull a little more gas out of it and I will be dialed in. https://docs.google.com/file/d/0ByL2EF4qKSUZbzNmMkQyRFVRalU/edit Link to comment Share on other sites More sharing options...
openclassterror Posted December 31, 2013 Share Posted December 31, 2013 Glad to help! I am much weaker in my pistol knowledge and I am glad I can pay forward the assistance I have gotten from the guys on the pistol forums. The combined amount of troubleshooting knowledge on this forum is far beyond what a gunsmith could attain in several lifetimes. It is like having a roomful of specialists on hand when you have a problem, and everyone pitches in! Just glad my little bit of knowledge helps now and again Link to comment Share on other sites More sharing options...
Onagoth Posted December 31, 2013 Share Posted December 31, 2013 When you put the gas block back on is it firmly seated against the shoulder? No, if had done so the holes would have been off by approximately .020. I measured the two holes from the shoulder and from the end of the block and placed the block accordingly. This seems unusual to me....I've only built a few uppers, but each one the gas block was bumped right up to the shoulder. I guess it also eliminates the advantages of an adjustable block when it has to be fully open Link to comment Share on other sites More sharing options...
openclassterror Posted December 31, 2013 Share Posted December 31, 2013 (edited) When you put the gas block back on is it firmly seated against the shoulder? No, if had done so the holes would have been off by approximately .020. I measured the two holes from the shoulder and from the end of the block and placed the block accordingly. This seems unusual to me....I've only built a few uppers, but each one the gas block was bumped right up to the shoulder. I guess it also eliminates the advantages of an adjustable block when it has to be fully open On a non- free float clamshell type fore end, there is a stamped sheet metal plate slightly cup-shaped to capture the front of the guards. It fits between the gas block and the shoulder on the barrel. Many lo-pro gas block manufacturers assume you are doing away with the cup, and position the port to align without it there. If the block takes into account the thickness of the plate, (but you don't use the plate) the port will be too close to the shoulder. Hence the "leave a gap" theory. However, on a carbine barrel the hole in the barrel is about .062 to .065, and the hole in the gas block is typically .159. So you could be off-center by almost .050 either way from perfect alignment without restricting gas flow. The guard retainer plate is under .030 thick, so you will only have a problem if one or both holes is out of location too. On longer gas systems the port gets bigger in the barrel, so your margin for error shrinks drastically. Tom Edited December 31, 2013 by openclassterror Link to comment Share on other sites More sharing options...
fastshooter03 Posted January 1, 2014 Share Posted January 1, 2014 OP-on your first post you wanted to know why a carbine buffer heavier than your's worked. The carbine buffer has a spring perch further forward than the rifle buffer so a small amount of spring was uncoiled. I see it's working for you now. If you build a similar upper I would suggest ditching the adjustable gas block for a steel unit and maybe play with a wolff Reduced Power buffer spring. Nick Link to comment Share on other sites More sharing options...
Thehotrodpig Posted January 1, 2014 Author Share Posted January 1, 2014 OP-on your first post you wanted to know why a carbine buffer heavier than your's worked. The carbine buffer has a spring perch further forward than the rifle buffer so a small amount of spring was uncoiled. I see it's working for you now. If you build a similar upper I would suggest ditching the adjustable gas block for a steel unit and maybe play with a wolff Reduced Power buffer spring. Nick Ya that got answered a couple posts after asking it. Makes sense once I actually looked at it. Link to comment Share on other sites More sharing options...
Onagoth Posted January 1, 2014 Share Posted January 1, 2014 When you put the gas block back on is it firmly seated against the shoulder? No, if had done so the holes would have been off by approximately .020. I measured the two holes from the shoulder and from the end of the block and placed the block accordingly. This seems unusual to me....I've only built a few uppers, but each one the gas block was bumped right up to the shoulder. I guess it also eliminates the advantages of an adjustable block when it has to be fully open On a non- free float clamshell type fore end, there is a stamped sheet metal plate slightly cup-shaped to capture the front of the guards. It fits between the gas block and the shoulder on the barrel. Many lo-pro gas block manufacturers assume you are doing away with the cup, and position the port to align without it there. If the block takes into account the thickness of the plate, (but you don't use the plate) the port will be too close to the shoulder. Hence the "leave a gap" theory. However, on a carbine barrel the hole in the barrel is about .062 to .065, and the hole in the gas block is typically .159. So you could be off-center by almost .050 either way from perfect alignment without restricting gas flow. The guard retainer plate is under .030 thick, so you will only have a problem if one or both holes is out of location too. On longer gas systems the port gets bigger in the barrel, so your margin for error shrinks drastically. Tom Thanks...learned something new today Link to comment Share on other sites More sharing options...
tgarza Posted January 11, 2014 Share Posted January 11, 2014 I had the same problem with a similar set up: 18" Criterion HBAR barrel, SLR rifle works DA7 adjustable gas block, JP low mass BCG,tuned spring and buffer. After I exhausted all options I drilled the gas port to .104( I worked my way up to that point) and now it runs like a sewing machine. Link to comment Share on other sites More sharing options...
TitoR Posted January 12, 2014 Share Posted January 12, 2014 Tgarza, what was the criteria to open the port to that measurement? GB adjustment range? Is adjustment more positive with the opened port? Link to comment Share on other sites More sharing options...
fastshooter03 Posted January 12, 2014 Share Posted January 12, 2014 IIRC Criterion barrels are match grade lapped barrels. Sometimes that type of rifling just won't have enough port pressure with less than .100" diameter. I had to open my Broughten 5C AR barrels up to number 38(.1015) or 36(.1065) drill. Can't remember right now. Nick Link to comment Share on other sites More sharing options...
openclassterror Posted January 12, 2014 Share Posted January 12, 2014 Good point. Lapped barrels can show a noticeable pressure drop on a piezo transducer measuring before and after lapping. Theoretically less friction and a slightly larger bore result in less peak pressure. Twist rate can also influence pressure, to a small degree. My personal thought on 18s and 20s with rifle length gas is to START with a port size of 0.1035 if I am using an adjustable gas block. It is easy to turn it down if the parts combo requires, and no need to take it apart immediately due to not enough gas. Mostly because I have to make money at this, and re-doing stuff prevents me from doing that. Link to comment Share on other sites More sharing options...
fastshooter03 Posted January 12, 2014 Share Posted January 12, 2014 Sounds good to me. Bigger hole with adjustable block would prob save a bunch of headaches for some people. Nick Link to comment Share on other sites More sharing options...
tgarza Posted January 13, 2014 Share Posted January 13, 2014 TitoR, I opened up the port gradually until I was able to get the gun to cycle and lock back on an empty mag. I started with a #39(.0995) drill bit and then tried a #38(.1015) niether of those offered results so I bumped it up to a #37(.104) and It runs now, the gas block is opened up about 3/4 of the way open. Link to comment Share on other sites More sharing options...
TitoR Posted January 14, 2014 Share Posted January 14, 2014 Tgarza, appreciate the information. Is drilling into a SS BArrel something that can be done with hand tools, or should i head to a local gunsmith? Link to comment Share on other sites More sharing options...
tgarza Posted January 14, 2014 Share Posted January 14, 2014 Im not sure about SS, I believe my barrel is chrome. I used a drill press with a drill vise and ordered the bits from Midway. If you dont feel confident I would seek the services of a pro. Link to comment Share on other sites More sharing options...
openclassterror Posted January 14, 2014 Share Posted January 14, 2014 Barrels in Stainless are about the same hardness as Chrome-moly, unless we are talking black star barrels. Should be about mid 30s Rockwell for a 416ss barrel. Use tapping fluid and a little less spindle speed than for 4140, and make sure to keep drill bit moving. If you stop feeding in the cut, work-hardening can result. Set your depth stop before starting, use a rigid setup, and feed steadily into the material. Don't advance too fast, or the burr you push into the bore will be hard to remove. If this description makes you uneasy then best to take it in. Barrels cost too much to replace to be worth saving 10 or 20 bucks if you don't have the right equipment. Link to comment Share on other sites More sharing options...
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