Chokes and velocity

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Dr. Kralenstein

Dr. Kralenstein

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Apr 14, 2022
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Ok, so how much drag does a choke add to a pellet, and is it worth the velocity drop for the possible accuracy benefits?

I know that's all relative to the specific gun. Reason I ask, I'm at a crossroads with my newly built 357 kral.

Ok, so my issue is I now have a giant cannon of a rifle, and it's producing "meh' power for running a 300 BAR fill, 23" barrel, and a heavy dual spring that is so stiff I feel like it will break something soon.

I'm only getting 750 fps max, and if I set the spring tension to anywhere near comfortable, it drops into the 550 range. Not worth it for what it is. I'd accept that power level if it was compact, and just make a nasty little short range squirrel thumper.

Anywho... I know this thing has a heavy choke, and it's damn accurate with the JSBs at 35 yards so far(haven't stretched it out yet), but im wondering if a tight choke is causing me to have such a low velocity? I'd think this gun would be in the 800s with this beastly spring set up and barrel length/fill pressure.

What are your thoughts?
 
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A tight choke can have a meaningful effect on velocity when the valve dwell is set very miserly, however that wouldn't generally be the case for a big bore.

Do you know what material the poppet is made of? If it's the typical Delrin or nylon, the excessive cocking effort could potentially be made much better by upgrading to PEEK or PEI.
 
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With a slug a choke can have a decent effect on velocity. Not so much with a pellet because of the lack of drag. But the loss with a slug while more than a pellet, is far less than what would even put you in the 800fps range. I believe your problem, if there is one, is in your air delivery. How is the porting all the way from the valve to the barrel? Is there meat left on the bone to decrease the poppet diameter?
 
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Only 750 FPS with JSB's on a 23" barrel does not sound right for a .357 @ 300 bar.


I'll assume its unregulated and plenty of volume available during the shot cycle that isn't heavily restricted?
Either the porting on that setup is abysmal or that hammer weight/spring combo is just not providing enough dwell.


-Matt
 
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nervoustrig said:
A tight choke can have a meaningful effect on velocity when the valve dwell is set very miserly, however that wouldn't generally be the case for a big bore.

Do you know what material the poppet is made of? If it's the typical Delrin or nylon, the excessive cocking effort could potentially be made much better by upgrading to PEEK or PEI.
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Whatever the factory poppet is. So most likely delving by the looks of it. I've never understood how a different type of plastic would make it more efficient or powerful. Is it that much lighter?

I have no valve spring in currently. With one, even a super light one, it drops in velocity.
 
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Stubbers said:
Only 750 FPS with JSB's on a 23" barrel does not sound right for a .357 @ 300 bar.


I'll assume its unregulated and plenty of volume available during the shot cycle that isn't heavily restricted?
Either the porting on that setup is abysmal or that hammer weight/spring combo is just not providing enough dwell.


-Matt
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Porting is about 6 or 7mm, I'd have to check again. There isn't much room left to port more, at least without a lathe or mill to be 100% confident I wouldn't booger it up.

The barrel and valve are at stock ports.. The passage thru the drop block I opened up a 1/16" which is about all I'd be willing to go without burning through the valve o-ring seat. I just matched the action port to those, which IIRC is 6.35mm or 1/4"
 
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Stubbers said:
Is this a caliber conversion or did it come as .357? Wonder what your hammer weight is in grams.

-Matt
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Conversion on a gun that only normally goes up to 25 cal. I could tear it down again and weigh the hammer, but it's decently heavy.

I haven't opened the throat of the valve up or trimmed the OD of the poppet. Didn't want to mess up the valve seat. Valve should be larger anyways, it's from a 357 gun.

I basically took a bighorn 357 barrel and valve/drop block and modified an NP03 action block to fit them. So I am running a shorter/lighter hammer than the bighorn. Guess that would mean less hammer stroke as well. The BH action is about 3/4" longer on the rear than the standard puncher blocks for the larger hammer.
 
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First, it would be helpful if you knew the specs of the barrel...bore & groove; but this is not absolutely neccessary. You will need a MICROMETER (NOT Caliper) to measure the diameter of your pellet. Some pellet/slug manufacturers give the precise measurement of the ammo in 100th of an inch (i.e. .216, .217, .218 for .22 caliber ammo). Larger calibers usually don't have as many size options. Measure the diameter of the head of that pellet and make note of it. What you want to do next is take that pellet, and push it through the barrel, rear to front, till it exists the front of the barrel. This means you will first need to remove the barrel from the breech, if possible and you feel inclined to do so. Measure the diameter of the HEAD of that same pellet after it comes through the barrel. THAT, is the diameter of your choke. What you want to do moving forward is purchase pellets that are sized by the manufacturer as close to your choke diameter as possible...OR, you can buy a pellet sizer in the size of your choke and resize pellets that are slightly off. This would be your best bet without and avoiding any hardware changes
 
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Dr. Kralenstein said:
Oh no, it's a decending shot string after the first mag, and only getting 21 shots out of a 500cc bottle before its already in the 700fps range.
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Yea major valve lock, should try to get more throw and a heavier hammer if possible. One can notch the face of the hammer where the sear engages to increase throw provided there is enough space behind the hammer, few other ways to skin the cat there too, if there is .2" or greater valve stem protruding one could shave it safely down to .2", or just run a recessed style striker.

-Matt
 
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Stubbers said:
Yea major valve lock, should try to get more throw and a heavier hammer if possible. One can notch the face of the hammer where the sear engages to increase throw provided there is enough space behind the hammer, few other ways to skin the cat there too, if there is .2" or greater valve stem protruding one could shave it safely down to .2", or just run a recessed style striker.

-Matt
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If it was valve lock, the velocity would increase as pressure dropped vs decrease with each shot. I would have way more shots per fill. It sure seems like I'm going through plenty of air, but not getting the velocity for the amount used.

Thats why I was thinking a tight choke was slowing me way down. Never had that happen before, but this is also a much heavier projectile then I'm used too in pellet rifles. No accuracy issues, so I know it's not clipping and slowing down from impact.
 
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Dr. Kralenstein said:
If it was valve lock, the velocity would increase as pressure dropped vs decrease with each shot. I would have way more shots per fill. It sure seems like I'm going through plenty of air, but not getting the velocity for the amount used.

Thats why I was thinking a tight choke was slowing me way down. Never had that happen before, but this is also a much heavier projectile then I'm used too in pellet rifles. No accuracy issues, so I know it's not clipping and slowing down from impact.
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You said any increase in valve spring even the lightest makes for less fps/fpe, which is indicative of the valve not being hit hard enough..

If you're unregulated at 300 bar with a .357 and 23" barrel, even with .15 porting you'd be well over 800 fps...so something is really fishy seeing a 23" barreled .357 at 300 bar be limited to 750 fps with 81 gr.

There could be a major port restriction at the throat or elsewhere robbing performance but I personally highly doubt its the choke or break out friction. Provided there is no blow by from the tp/breech, I would have to assume its due to the valve arrangement, or hammer arrangement.

-Matt
 
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Stubbers said:
You said any increase in valve spring even the lightest makes for less fps/fpe, which is indicative of the valve not being hit hard enough..

If you're unregulated at 300 bar with a .357 and 23" barrel, even with .15 porting you'd be well over 800 fps...so something is really fishy seeing a 23" barreled .357 at 300 bar be limited to 750 fps with 81 gr.

There could be a major port restriction at the throat or elsewhere robbing performance but I personally highly doubt its the choke or break out friction. Provided there is no blow by from the tp/breech, I would have to assume its due to the valve arrangement, or hammer arrangement.

-Matt
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I'm leaning towards hammer weight and stroke. The valve stem on these guns only extends out the back about a 1/4".

All my ports are lined up and no leaks, it has to be hammer weight. A stock bighorn will do close to 800 with the weak single spring they have. A little work, and I've heard 900 is possible.
 
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Took the action apart again and measured. The stock ports on the barrel and bighorn valve are 6mm. I currently have the block and another valve off a jumbo ported to 1/4 or 6.35mm

I turned the jumbo poppet down to match the bighorn poppet which is smaller OD. The throat was about 7mm on the bighorn valve, and I ported the jumbo one to 5/16(8mm). The internal volume of the bighorn is actually smaller than the jumbo valve.

I then ported the bottle adapter piece that goes between the drop block and bottle. Stock hole was 6mm, it is now 8mm. The thru hole to the valve I cannot go any larger, it's currently a loose 1/4", but can't get the next bit in it all the way. So probably 6.5-6.6mm

I'm going to cut down a 1/2" drill bit and stuff a section into the spring pocket on the hammer to add some weight. I will probably port the barrel to match the rest of the 1/4", but there isn't much meat there before o-ring groove, and I don't want to try ovaling all the ports just yet.

I feel the throat opening and extra hammer weight will get me somewhere. Larger internal volume in the valve should help as well.

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I was going to just swap the BH valve poppet and stem into the jumbo valve, but it's just slightly larger OD stem. So that means the jumbo valve stem will be lighter. Hopefully it doesn't break!

Another thing to note. The throat on the BH valve (on the left in the pics) doesn't stay the same size all the way to the exhaust port. I drilled all the way to the end of the the throat, but not into the stem seal area.
 
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