Velocity Difference , Piston/DI systems.

[survivalshop]

  It was an interesting day at the range , I actually thought this one test would have come out different , it just opened new questions , in a way.  Look at the Chrono. results between two 16" Mid Gas systems rifles , one is a Superlative Piston set up the other is a DI gas system .Two different twist rate Barrels , a 1-8 & a 1-7 , two different manufacturers & barrel Contours , one an Match SS Light weight Barrel , the other a Gov. weight , Nitrated Barrel . Both have Free Float Hand guards. 

Rifle # 1 - POF Billet Receivers , Wilson Combat , SS match 1-8 Barrel , Mid Gas , DI , 16" BBL ( $ 235 )., Bushnell 1x4 AR Optics Rifle Scope w BDC reticle . ACT Trigger 

Rifle # 2 - Tactical Machining Receivers , Mil Spec., Brownells Gov/ weight , Nitrated , 16" Mid Gas  . 1-7 twist rate Barrel  ( $ 89. ) ./w Superlative Mid Gas Piston assembly , Burris 2x7x35 Fullfield  E1 Reticle , Rough Justice Trigger .

Ammo - Hand loads , 62 gr. FMJHP w/Cannelure , 23.5 gr. TAC Powder, CCI Primer , .223 brass . I have hotter loads , but didn't shoot them this time .

 

  Rifle # 1  (10 shot )

  Avg . Velocity               ES                       SD

    2661 fps                    136 fps               44

 

   Rifle # 2 ( 10 shot )    

 

   2636 fps                       56 fps                19 

 

 

    So , the match Barrel looks like it doesn't like this Loading , but both shot very well . I don't get the Chrono results & the Target results , but maybe it doesn't matter, Any thoughts on this ?

 Twist rate , Piston system ? 

  Good question here or should I say preliminary results , is the DI Rifle only has about on avg. 25 fps over the Exhausted Gas Superlative Piston system . More testing is needed with other loadings I have , on another date , but 25fps is not much , so evidently , this piston system where excess gas is exhausted out or Bled out instead of just valved off to only let enough into the Gas Tube to cycle the Action sufficiently .

  Both Barrels are relatively new , very low round counts  .I'm pleasantly surprised how well they both shot , the Piston rifle with the Rough Justice Trigger had the edge with a better Scope & Trigger & it does show in the group size , but as I got used to the ACT Trigger , the rifle shoot well @ 100yds. I shot the # 1 Rifle first & sighted the Scope in , let it cool while I tested the # 2 Rifle .

    

Edited December 1, 2017 by survivalshop

[HotRod308]

Although I have no way to prove it..... I have found that DI system with a lot of drag from the gas rings results in a little higher FPS while with little to no drag from the gas rings results FPS are a little slower.

As carbon builds up on the gas rings, the ES increases as the gas rings cause more drag from when they are cleaned.

The gas piston might be smoother as it has no gas rings and the BCG has very little drag when moving compared to a BCG with gas rings.

I know many people don't agree with my idea of DI system without gas rings, and that's fine. Like I said I have no way to prove it, it just my thought.

[Robocop1051]

I would think the FPS difference would be from the 1:8 vs the 1:7. There is a lot less drag/friction in the 1:8 barrel. I know I used to get screamed at as a kid for shooting 55gr through the old mans HBAR match too fast. He was always afraid I’d burn out that old barrel.

Is that a normal speed? That speed seems a little slow, even for the 62gr. I remember when Jgun was loading 125gr .300Blk to 2500+ FPS. Just seems like a 62gr should be well over 2600. 

[Sisco]

13 minutes ago, Robocop1051 said:

I would think the FPS difference would be from the 1:8 vs the 1:7. There is a lot less drag/friction in the 1:8 barrel. I know I used to get screamed at as a kid for shooting 55gr through the old mans HBAR match too fast. He was always afraid I’d burn out that old barrel.

 

My “old” HBAR match barrel AR-15 is still going strong after 3,000 mixed 55, 62, and 75 grain bullets.I don't think your Dad had to worry. Still-shoots tight groups. Haven’t done many mag dumps with it though.

[308kiwi]

I'd tend to agree with Robocop, the speed difference is more likely from the different twist rate than the difference in gas systems but not really any way to prove it I guess.

 

[survivalshop]

9 hours ago, Robocop1051 said:

Is that a normal speed? That speed seems a little slow, even for the 62gr. I remember when Jgun was loading 125gr .300Blk to 2500+ FPS. Just seems like a 62gr should be well over 2600. 

     I had 62 gr. Bul;let loads with me that break the 3100fps speed from a 24" 1-10 Barrel , didn't try them , just wanted to test to see if the Exhausted Gas Piston system would reduce Bullet speed in any significant way ,  which in any case is not much , so to me its insignificant which gas system you are running , as far as Bullet speed differences .

   The TAC Powder loads were a test batch of lower powered loads , working up , I had 24.0 gr. , 24.5 gr. & I think 25 gr. Loaded with me , not to mention some other Loads with different Powder & loadings . I will Chrono them at another time , I think it will interesting to see if the FPS span changes with higher velocities , but I doubt it will change much .

5 hours ago, 308kiwi said:

I'd tend to agree with Robocop, the speed difference is more likely from the different twist rate than the difference in gas systems but not really any way to prove it I guess.

 

      Sure there is , all I have to do is swap out Gas Systems & Bolt Carriers & Upper receivers , the Piston system has a Bushing in the Gas Tube Hole on the Upper Receiver . I see no reason to even do that , there is not much difference in FPS spread to even do any further testing IMO .

    Even a Adj.Gas Block would not make much of a difference ,IMO also , well maybe if you closed all gas to the Gas system , but that would be another story .

  I was just curious , because it has been discussed before or brought up . 

[survivalshop]

     I tested the two Rifles again today with Ramshot , Xterminator with a higher velocity loading of 24.6 gr./ 62 gr FMJHP & WW primer . Completely opposite results in the Average Velocity with the higher velocity ammo , which is going to be my new loading , because I like the way it shoots in both of these 16" rifles , but also the 24" .   (10 shot test )

 DI or PISTON                  AVG. Vel.             ES                     SD

    Piston                           2864 fps             59 fps              18 fps

    DI                                   2821 fps             69 fps               21 fps

      I didn't expect the Velocity to switch with the Piston System , I thought the avg. spread would have grown on the DI system side not switch , because the Piston system is a bleed off type , where the excess gas is expelled into the atmosphere , not down the barrel behind the Bullet . 

     The ES & SD did square off better , more consistent ammo & both Rifles shot really good in the gusting wind conditions & the fact that the Piston Rifle has a Rough Justice Trigger & the DI Rifle has an ACT Trigger in it  & Scopes are different , with the Piston Rifle having a better & more magnification then the DI Rifle . The ten shot groups were in the same place , impressive . 

      

 

[survivalshop]

    Everything being the same with the load , weighed charges , same bullets , same COL , Same Primers , same Brass ( LC ), two twist rates , side by side comparison . Why would a a 1-8 be faster then a 1-7 at one velocity , but slower at a higher velocity  , they were different Powders used  , but why would that have anything to do with it . The side by side comparison in the first test with TAC Powder the 1-8 avg. was 25 fps faster then the 1-7 twist , then with a different Powder ( Xterminator ) with a larger Powder charge , do a 180 & switch the 1-7 having an avg. velocity of 43 fps faster then the 1-8 twist .

  Is it the Piston system with the atmospheric discharge using less gas to operate the Action properly or efficiently  ( because its adj. ) meaning more gas is actually staying in the Bore , behind the Bullet , then the DI system that is fixed orifice & the wasted or excessive gas is going into the Upper Receiver ?  

  Both Barrels are 16" , Mid Gas systems , the 1-7 is Nitrated  & the 1-8 is Stainless steal , both about the same round count . ( low ) 1-7 a Piston system. 1-8 a DI system .
 
    Thoughts ?

[survivalshop]

  I think the answer is Powder Burn Rate & it was explained in the Thread I posted here in the reloading section for the M14 .

[bubbas4570]

Would the milliseconds of the piston resistance be something that is helping with the additional gas (residual) pressures?

This is totally uneducated, but common sensical, thought....which is more than likely wrong. But I just had to throw it in.

[survivalshop]

14 hours ago, bubbas4570 said:

Would the milliseconds of the piston resistance be something that is helping with the additional gas (residual) pressures?

This is totally uneducated, but common sensical, thought....which is more than likely wrong. But I just had to throw it in.

  It could be , I'm searching for an answer , the Burn rate of the two powders sounded good , with the Powder peaking at a certain point in the barrel made sense , till I thought about it , but now I'm not too sure . 

 What I have to do is run the Ramshot Tac up to the same velocity as the Xterminator & see how that comes out . I have different loading already made up for testing & should have no problem reaching it . The Load for that first comparison was the lowest loading I started out testing . I really like that Xterminator load .

  Xterminator is a faster burning rate the Tac , but very close to each other , from what I see on the burn rate charts .That may be why the 16" , Mid length gas systems like it , but will not know that till I reach the same velocity to match the Xteminator .

  

[survivalshop]

 I looked at my Chrono results for Tac loads that come close to the velocity of the Xterminator  & the 25.5 gr. of Tac was close to the Xterminator @ 24.6gr. from my 24" AR , so I will test that ( since I have a bunch already made for testing ) in the 16's to see if there is a difference . I also gave some WC 846 loaded to throw in the mix .

  I also have some 25.5 Tac loads with both Remington match & CCI primers to see if there is any difference , just for the hell of it .

Edited January 28, 2018 by survivalshop