Tuning Barrel porting on 25 cal question

[Airgunmann]

How large can you go with a round port before the pellet nose would dip down and cause damage to the pellet I'm wanting to enlarge the barrel port to the transfer port size in the block of a older raw hm 1000 x 25 cal shooting 33.95 mk 2 jsb pellets barrel port is the choke point on it the valve port is over 6 mm and I'm not sure how big the block porting is but I know it's larger than the barrel port will to large of a transfer port hurt the extreme spread or doesn't that matter with a regulated gun thanks for any input 

 

[nervoustrig]

With pellets you can go with 75% of the caliber, 80% tops. So for .25 cal you’re looking at 0.25 * 0.75 = 0.1875” which is a 3/16” drill. That is a very common size for DIYers to use on a .25 cal.

With long slugs you can cheat a bit larger because they aren’t as apt to dip into the barrel port and get scarred while being seated. However it’s not hard to grind an oblong port with a cheap carbide burr in a Dremel so for peace of mind it’s best to stick with a 75% starter hole.

 

[Airgunmann]

Yeah agreed I'm not looking for big power just want to open it up some without having to report the block I've went bigger in the past and oblonged the holes but couldn't remember the numbers I went to in the past 

 

[thammer]

I run a 13/64 still bit on my high power tunes. That's .203" and have no issues. I can easily break 60fpe with that porting. You can also use a countersink bit to make a funnel design in the barrel if the port in the receiver is still larger than the port on the barrel. 

 

[triggertreat]

You can oblong the 0.1875" drilled port more. I have my .25 Marauder opened up to 0.220" with no issues. Just stay out of the lands and grooves. You can also cut a star-like configuration instead to gain more without issues.

 

[Long_Gun_Dallas]

For me, .21" would be the max. We will call it .20 to be safe. I always radius the inside of the port edge and elongate it. This makes hanging pretty much impossible. But as triggertreat stated, be careful not to cut into the rifling. 

 

[Airgunmann]

I'll probably go to .200 ish for it and do some tapering on the inside so if it does dip some it won't damage it . I can get to 60 fpe all ready just want to open it up so maybe it doesn't have to be pushed so hard to get there. Just hope the extreme spread doesn't change or is that only when guns are unregulated that it changes 

 

[nervoustrig]

Large porting will have no negative effect on the extreme spread in a regulated rifle provided you have the hammer spring tension dialed in properly.

In an unregulated rifle, it will produce a narrower bell curve because it trades shot count for higher energy.

 

[Long_Gun_Dallas]

Yep, I am real warry of porting barrel inlets on unregulated guns. A lot of times the port size is optimized for consistency 

 

[triggertreat]

I agree on the ES getting out of control with larger unregulated ports. I wouldn't open one up very much or any unless regulated.

 

[Airgunmann]

Porting of the original was at like .178 took it to .200 and radius the edges with a Dremel until I got no pellet damage took a a couple tries to get it there but I believe it's good now I'm going to make a peek poppet now does anyone remember how much over lap on each side of the valve I would need with peek original poppet is .350 and valve throat is .265 

 

[nervoustrig]

PEEK's compressive strength is in the ballpark of 20,000psi. That's what we need to stay under.

For a throat of 0.265" diameter (0.0552in^2), let's assume a poppet with 0.010" overlap for a diameter of 0.285" (area=0.0638in^2). The difference in area is therefore 0.0638 - 0.0552 = 0.0086in^2, which represents the annular ring of PEEK supporting the force acting on it. So for example if the regulator's setpoint were 1800psi, the force acting on the poppet is 1800psi * 0.0552in^2 = 99 lbs. This 99 pounds is supported by an annular ring with an area of of 0.0638 - 0.0552 = 0.0086in^2. So we have 99lbs of force acting on an area of 0.0086in^2 which means the PEEK is seeing a compressive force of 99lbs / 0.0086in^2 = 11,512psi. That leaves you with a design margin of almost 2:1 relative to PEEK's rated compressive force of 20,000psi. So you could probably get away with a slightly smaller sealing margin but that's probably a good place to be from an reliability standpoint. 

 

[Airgunmann]

155 bar is reg setting I was thinking .305 for size or .020 over lap on both sides or is that not enough 

 

[nervoustrig]

155 bar (2248psi) changes the poppet's holding force to 124lbs. With 0.020" sealing margin, the area of the annular ring of PEEK carrying this force is 0.0731 - 0.0552 = 0.0179in^2. Thus the PEEK will be seeing a compressive pressure of 124lbs / 0.0179in^2 = 6927psi which is only about a third of PEEK's 20,000psi rating. Should be plenty.

 

[Airgunmann]

Ok thanks for helping me out with all the numbers 

 

[Airgunmann]

Got the peek poppet made to .300 made the valve stem same length and sizes glued on the poppet and lapped it the valve installed valve and poppet checked it and it sealed at 150 bar had a lite spring installed and fired off a couple shots it started to leak kept firing and it sealed up after about 3 shots and stayed sealed down to under a 100 bar had like 50 or sixty shots between 150 to under a 100 and over 95 percent of the time it was sealed up I filled up to 3500 psi and tested the lite spring made 400 fps the original gun spring made 700 fps and my heavy spring made it to 954 fps on a ssg had to shorten the hammer striker about a 1/16 of a inch to get there using 34 gr mk2 jsb pellets I'm pleased I can detune to 900 fps instead of being maxed out at 899fps like I was before reg is set at 155 and it's a huma 

 

[Airgunmann]

Thanks for all your input everyone