FPE Loss from the moderator

[rangur1]

have not had time to do full range tests with chrony on all my guns. this was just one instance where i chronied my BSA R-10 SE 22 cal. with the factory barrel shroud with fused moderator the gun produced 26 FPE by pushing a 18.1 gr JSB an ave of 810 fps. when i removed the factory shroud setup it was 840 fps for about 28 FPE. next i put on a donnyfl shogun mod and pellet speed dropped to 785 fps for about 24 FPE the last was changing to a donnyfl emporer v2 mod and the results were 745 fps and 22 FPE! Of course there were corresponding POI shifts as well. I don't understand the physics involved, but i always heard moderators make the projectile go slower. to chart it:

naked barrel 840 fps for 28 FPE

factory shroud 810 fps for 26 FPE

donnyfl shogun 785 fps for 24 FPE

donnyfl emperor 745 fps for 22 FPE 

 

[Bob_O]

That's weird. I'll ha e to try my Ultimate Sporter with just the shroud....it'll be pushing 1,000fps with JSB 15.89gr if same results as you have.

 

[elh0102]

That's weird. I'll ha e to try my Ultimate Sporter with just the shroud....it'll be pushing 1,000fps with JSB 15.89gr if same results as you have.

Bob, almost every air rifle I have seems to shoot best in the 850-900 fps range, and the 16 JSB is my favorite pellet. Your velocity sounds a bit hot, but maybe it shoot the 18's best. What have you seen between these two? 

 

[oneshot61]

That’s odd. I have a sumo on my rainstorm.30. I need to check the speeds with and without it. I was not aware the moderator’s had that affect on speed. I’ve heard of poi shifts due to clipping, but not slowing down. Maybe if you get some others to try theirs with and without their moderator you can get some confirmation as to what’s happening.

 

[Smitty911]

Interesting, I would assume the design of the LDC pushing the air around causing turbulence and adding some additional drag on the pellet, Makes intuitive sense to me. Not sure how to correct it unless someone has a Cray computer laying around and can minimize the turbulent airs effects on the pellet.

Smitty

 

[Bob_O]

That's on Max power. I can dial down to the speed range I want for the different weight pellets.

 

[BigTinBoat]

Interesting, I would assume the design of the LDC pushing the air around causing turbulence and adding some additional drag on the pellet, Makes intuitive sense to me. Not sure how to correct it unless someone has a Cray computer laying around and can minimize the turbulent airs effects on the pellet.

Smitty

Gotta wonder, if the pellet is in "front" of the air, how is the turbulence (caused by the LDC) effecting the pellet, since the pellet is already in front of it?

 

[John "spysir" Blount]

Anything using a proper air stripper will NOT have air in front of the pellet.

If you tried to design one to give all the air blast you could to the skirt after leaving the barrel I believe you could see a fps jump - and likely crap groups-

Unless using wipes there not slowing pellets down.

Firing through the chrony level & square & same barrel and distance all all else exactly = ? Shoot from a bag/bench & put your chony out maybe 5 full feet ( yards is better) and see if the difference isnt a lot closer.



John

 

[davidsng]

I lost velocity once after an installation aof a LDC. A seal blew, it was on a semi auto, may have been from the extra pressure from the gas not escaping. Just an idea.

 

[Smitty911]

Interesting, I would assume the design of the LDC pushing the air around causing turbulence and adding some additional drag on the pellet, Makes intuitive sense to me. Not sure how to correct it unless someone has a Cray computer laying around and can minimize the turbulent airs effects on the pellet.

Smitty

Gotta wonder, if the pellet is in "front" of the air, how is the turbulence (caused by the LDC) effecting the pellet, since the pellet is already in front of it?

There is the volume of air inside the barrel from the muzzle to the front of the pellet, all that air will need to move, which it does, I would assume there is some compression as the pellet is moving that air first. That air hits the LDC first and starts to become turbulent and could be causing the decrease. Any aerodynamicist around?

Smitty 

 

[steve-l]

Interesting, I would assume the design of the LDC pushing the air around causing turbulence and adding some additional drag on the pellet, Makes intuitive sense to me. Not sure how to correct it unless someone has a Cray computer laying around and can minimize the turbulent airs effects on the pellet.

Smitty

Gotta wonder, if the pellet is in "front" of the air, how is the turbulence (caused by the LDC) effecting the pellet, since the pellet is already in front of it?

There is the volume of air inside the barrel from the muzzle to the front of the pellet, all that air will need to move, which it does, I would assume there is some compression as the pellet is moving that air first. That air hits the LDC first and starts to become turbulent and could be causing the decrease. Any aerodynamicist around?

Smitty

You are exactly correct. The more effective the moderator is, the greater the speed loss. I initially thought that the moderator was attenuating the pellet speed, but that's not the case. It is the column of air that must move out of the way allowing the pellet to accelerate to its exit velocity. It is the additional resistance the moderator provides on the column of air in FRONT of the pellet that impedes the pellet from achieving the same velocity as without the moderator.

POI change is also common, but usually not because of the pellet hitting the moderator. The cause is due to the droop effect on the barrel because of the additional weight of the moderator and it usually does not change the gun's accuracy. The gun will group the same, just not in the same place.

 

[JeffDodge]

Chronied mine this morning and the Donny Tanto I have on there does not change the fps at all

 

[Imold]

I get no change in FPS with my Bully having the DonnyFL Emperor on it.

 

[ackuric]

Moderators won't dramatically change fps...unless you're clipping on them, or not adjusting the length from chronograph comparing it to naked / different ldc...

Certainly not 100 fps worth...



I have shot with roughly 6 different sized (both OD and length) moderators and never had any perceptible change in FPS...UNLESS I had clipping, even the slightest that leaves just lead on the baffle and not damage...JMO

 

[steve-l]

Moderators won't dramatically change fps...unless you're clipping on them, or not adjusting the length from chronograph comparing it to naked / different ldc...

Certainly not 100 fps worth...



I have shot with roughly 6 different sized (both OD and length) moderators and never had any perceptible change in FPS...UNLESS I had clipping, even the slightest that leaves just lead on the baffle and not damage...JMO

On my .25 Crown I lose 40 fps from with to without. There is no clipping. With the shroud moved all the way to the rear most position and locked, the without moderator speed is 940 fps. Just screwing on the moderator to the shroud reduces the pellet velocity to 900 fps. This test is very repeatable with identical results.every time. My moderator is very effective in reducing sound.

 

[ackuric]

Moderators won't dramatically change fps...unless you're clipping on them, or not adjusting the length from chronograph comparing it to naked / different ldc...

Certainly not 100 fps worth...



I have shot with roughly 6 different sized (both OD and length) moderators and never had any perceptible change in FPS...UNLESS I had clipping, even the slightest that leaves just lead on the baffle and not damage...JMO

On my .25 Crown I lose 40 fps from with to without. There is no clipping. With the shroud moved all the way to the rear most position and locked, the without moderator speed is 940 fps. Just screwing on the moderator to the shroud reduces the pellet velocity to 900 fps. This test is very repeatable with identical results.every time. My moderator is very effective in reducing sound.

I have very effective moderators as well, that reduce 10 db...doesn't mean I get a change in velocity...something isn't right if thats what you get is all I am saying. I've ALWAYS corrected velocity losses from one moderator to another by removing the condition of clipping pellets. YMMV but thats mine. Plus I'd be upset if changing my moderator around changed its POI...which 40 fps WILL do...FWIW I have an extend-able moderator on my custom air gun and it does NOT change POI I can still hit in the same hole in either position...YMMV but thats my mileage. My barrel is fully supported so the extra 4 ounces moving out 4 inches makes no difference nor is FPS loss present, yet from collapsed to extended I get an extra 4-5 db reduction..

 

[Coldair]

My best guess is that the end of the moderator is adding more distance to the Chrony sensor as compared to the naked barrel and the longer the moderator such as an Emperor, it is nearly a foot farther away as compared to chrony from the end of the naked barrel thus the lower fps. AND/OR it could be clipping the baffles effectively lowering the speed as well.

CA