MickB

Remember, the numbers are based on ASSUMPTIONS. The point of the exercise was to show that by the time the projectile has moved past the gas port and exited the barrel, the gas pressure in the barrel has dropped before the bolt unlocks. Projectile time in barrel past gas port ~ 0.00015 seconds Even allowing extra time for gas pressure in the barrel to dissipate, you should be at 0.001 or so seconds. REFERENCE (it's for 6mm PPC, but shows an example) http://www.frfrogspad.com/intballi.htm Another interesting reference. http://www.sadefensejournal.com/wp/?p=1093 Note that bore pressure ranges from ~6,000psi for a 20' barrel to ~11,500psi for a 10" barrel at the point the projectile exits the muzzle. Here is a reference from US Army tests that show barrel pressure drops to zero in 0.003 seconds in the M16 family (OK, so I was a little off. Sue me for using numbers from the top of my head! :-) ) http://www.armalite.com/images/Tech%20Notes%5CTECHNOTE104SEPT042014.pdf Now let's take the 0.05 seconds cycling assumption. If I use a 6-800 rpm cyclic rate (closer to the real deal), that number will be closer to 0.07-0.1 seconds Even parsing that into time for the bolt to mechanically unlock, you should still be at least one order of magnitude longer time for the bolt to unlock versus the bullet leaving the barrel and pressure dropping to ambient levels. Now you start to see a glimpse of what the designers are playing with. You can mess with reciprocating mass, spring rates, mechanical locking systems, etc. to retard the rearward movement of the action. The purpose is all the same - to make sure that pressure in the bore has dropped to a safe level before that bolt is unlocked I'm sure Mark will be along with more in depth numbers... :-) Mick Sorry Mick. Your math doesn't hold up. You are assuming that the pressure in the barrel drops from over 10,000 psi to +-14.7 psi instantly. The graphs I have seen indicate it takes longer for the barrel pressure to drop to ambient pressure after the bullet leaves the barrel than the time the bullet is actually in the barrel. Also the .05 cycling time would also include extraction, ejection, bolt slowing down from the effects of the buffer spring, bolt coming to a stop at the rear of its travel, the spring overcoming inertia, bolt accelerating, being stalled as it picks up the next round, bolt tripping the auto sear as it comes into battery, hammer spring overcoming inertia, hammer accelerating and striking the firing pin, firing pin being driven forward striking the primer, primer ignition, powder ignition, bullet acceleration and traveling down the barrel to vent had into the gas tube to start the process again. How much of that .05 seconds includes barrel overpressure which may effect extraction? Don't know.

LOL! you are correct on the roller locked action. The bolt carrier is forced rearward while the bolt rollers are forced to the barrel extension by the high pressure blow back until pressure has dropped in the barrel. Because the carrier is still moving rearward, by the time it has reached the end of its free travel, the rollers have disengaged and the whole assembly continues to move rearward. Timing and burn rate and brass quality are everything on this type of system. Wrong burn rate in some ammunition can screw up the timing and over pressure the system, leading to rims being torn off cases. Mick Smart arse... bugger off back to Australia Even with the delayed roller locking system, the cycle of unlocking starts at the moment of firing. The rollers delay unlocking until the pressure has dropped and the bullet has exited the barrel. The momentum imparted to the mass of the bolt then completes the cycle and extracts the case. The key is that the momentum starts before the pressure has dropped.

Let's do some basic math. There will be some assumptions for ease of calculation. 20 inch AR barrel = 0.5 meters (0.508 m for those that want to be exact) M193 55gr projectile muzzle velocity = 3,280 fps = 1,000 metres/sec Projectile spends 0.0005 seconds in the barrel (yes, I am ignoring acceleration, but I am keeping this simple, remember). Now, when the bullet just goes past the gas port, it has about 6 inches (0.15m) of travel left before it exits the muzzle. This means there is 0.00015 seconds until the bullet exits the muzzle and the pressure in the barrel drops back to atmospheric pressure. During that time, the gas has travelled down the gas tube and put pressure on either a piston or bolt carrier to impart kinetic energy to move it rearward. The bolt is still mechanically locked at this point. Now, let's assume a cyclic rate of 1,200 rpm (yes, I know AR cyclic rates are lower but bear with me). That is 20 rounds per second, or 0.05 seconds per round. Where am I going with this? The bullet has left the barrel BEFORE the bolt unlocks, meaning the pressure in the barrel is back to atmospheric and it is the rearward momentum imparted to the piston or bolt carrier (from the gas) that cycles the action. Yes, there is blowback pressure on the case while the projectile is still in the barrel, but that should be gone once the bullet has left the barrel. Mick

Are we forgetting the CETME/HK roller locked action that relies on blow back? No gas piston/tube at all on these designs and fluted chambers to help extraction.... Mick

Turn the mag tube in till it bottoms out on the nut, then back it off a quarter to half turn. The mag tube is now free to "float" and align with the barrel when you put the clamp assembly on. This means the mag tube will then align with the barrel. if not done, the clamp can pull the barrel into alignment with the mag tube and your POI will be off. Mick

Place a white paint line on the tube and nut so you can see at a glance if the tube has moved. Also line the inside of your clamp (or outside of your tube) with a layer of electrical tape - this provides extra friction to stop the tube spinning under recoil.... Mick

^^^^ What JJ said. Just tighten it up snug when reinstalling. Since it is blended, it is already timed as well..... Mick

The best way to dampen recoil on a 9mm AR is to go with a heavier recoil spring and a hydraulic buffer... Mick

Exact same behavior with my JP15. Shot fine and locked back at 80 degrees, but testing the next morning at 55 degrees caused it to be a single shot till I turned up the gas. This was with factory IMI M193 5.56. Interesting observation with a .308 G2 and using Australian ADI F4 ball ammo. This rifle has an SLR SA7 adjustable gas block. My belief is that the lower temps are causing lower gas pressure to cycle the action (PV=nRT), ie. gas volume is directly proportional to temperature. Here's the interesting part - the .308 performance didn't change at all at lower temps. Why? Australian powders were developed to be relatively temperature insensitive due to the climatic variations across the country and to maintain performance with the L1A1's adjustable gas system. This turned out to be the case with my G2 and lower temperatures. The same powder is loaded in the Australian Outback range of .308 and .223 that is sold here in the USA. It would be interesting to see if the same effect could be made with that series of ammo in a .223/5.56 rifle. If so, the next question is where can I buy the powder for my reloads! Mick

Patrick, I like the concept above, but I would suggest instead of 1 hit anywhere, it be modified to 1 hit in A/B/C zones, with one D being considered an FTN. It should reduce the spray factor a little and make the shooter think about whether they need that backup shot or not... Mick

The P-mags always functioned. The issue is with bolt lock back. Insert the steel DPMS mag and lock the bolt back and look through the ejection port and note how much of the bolt face the bolt catch covers. Repeat for the P-mags and note any difference. If you could post a pic of that comparison, it would be helpful... Mick

Eric, you know you can SBR your AR and get a Stag left handed upper, right? :-) Mick

As far as I know, there have been no changes at DPMS. If there is a change, I'm betting its' been a quiet one at Magpul... Would have to get a new batch of P-mags to try out... Mick

Try $50 for a stripped lower receiver at Brownell's or AIM.... Mick

DPMS are not making revisions as they consider it a Magpul issue. this was from my conversation with DPMS last week... Mick

While Roulette Tactical was still making stuff for shotguns, we designed and made a few examples of one piece end caps (+1 and +2) for the Versa tube. The first prototypes would loosen under recoil, allowing the sleeves to come loose, which would then allow the one piece tube to begin to spin out of the receiver. What was happening was that the recoil forces of firing were overcoming the frictional force of the flat end of the extension bearing on the flat end of the carbon fibre sleeve. What did we do? We cut teeth into the extension which meant an increase in frictional force bearing on the sleeve (lower surface area means higher force). After that, the extensions didn't move at all. Alas, there are only a literal handful of these Versatube extensions in existence. I still run mine with no issues. Maybe Mark can do something similar but in reverse? Cutting teeth into the end of the Versatube sleeve should achieve the same effect.... My guess is the RNT fix is a spring washer. Could also add a groove in the end of the Versatube sleeve to accommodate a spring washer for a cleaner look. Mick

A good chrome lined barrel will shoot plenty accurate for 3 gun. Look at the Criterion barrels, as an example. My first Tac Limited rifle was built using a Colt M16A2 barrel I had lying around, and it will easily drop Larues at 500+ yards. The key point here is GOOD chrome lined barrel and CONSISTENCY. It is much more difficult to get consistency through the bore when you add a chrome lining, meaning you will get some great shooting barrels within a batch of mostly average barrels. FN is known for their military heritage as well as their barrel making ability, so this fits in with their background. If you look at their SPR bolt action precision rifles, they have chrome lined barrels with a 1 MOA or better accuracy guarantee. So if FNH is doing something similar with their new competition rifles, then they should be plenty good for 3 gun. It also enables marketing of the same product to the "tactical" crowd as well. I'm sure we will find out soon enough just how good these rifles actually perform... Mick

I couldn't tell you what POF did. All I saw was the catastrophic results first hand. I would hardly expect that to be covered under warranty. Now, if the lower were marked "multi caliber".... Don't laugh, there are people that are stupid enough to think they can fire anything through a multi-caliber marked AR.... Mick

Yep. While touring POF's new facility a couple years ago I was shown a blown up rifle that had been sent in for "warranty" by an irate customer. Seems there was a .300BLK case still stuck in the chamber. Ooops! There ensued a discussion which is exactly what is described in the article and was confirmed by POF's own testing at the time. .300BLK in 5.56 chamber = bad juju... Mick

Aero Precision makes one just like that with the port door (my preference), as another option... Mick

Is that a winning combination even for a Patrol Rifle? Mick

Since you are looking at cost, go with a non adjustable gas block and standard BCG. M16 or AR15 types makes no difference, as it will be semi-auto only anyway. Also don't buy into the coating hype - the regular parkerized BCG's are just fine. Stick with a basic mil-spec charging handle - you can always add an extended latch later if you want to. As for the brake, just get the real deal JM. They are cheap enough anyway (~$40) and you know what you are getting. For the optic, best bang for buck is the Aimpoint PRO for Tac Limited or the Burris MTAC 1-4 in Tac Scope. Also, spend the coin on a decent trigger. Since you're building, get the JP classic trigger and cut the tail off your mil-spec hammer.... Mick

Sounds like not enough bolt velocity. for the action to close. Could be a number of things: - Is the gas block properly aligned and did it move at all? - Are you sure the gas adjustment screw hasn't moved? - Was there a temperature change between when you adjusted the block at the range and then shot it in a match? Case in point. I tuned a JP 15 here in AZ on an 80 degree afternoon. Next morning, at 50F, not enough gas to cycle the action. Mick

If you are going to use the rifle for 3 gun as well as a suppressor host, then yes, the Surefire brake is the way to go. Mick

Kurt, I think you have a broken disconnector on your keyboard... Mick