.300 Whisper Pistol Build

[abyssdncr]

KS finally got Class III approved by our illustrious Governor, so I'm in the process of converting my 16" 300/221 upper into an 8" upper with the intent of ending up SBR & suppressed. Right now it's hanging out on my "Punisher" pistol...



Plan on keeping the YHM rail with a low pro gas block and adding one of the adjustable gas tube valves to a pistol length gas tube to adjust the gas. Made it out to a friends place this week and chopped, turned, and threaded the barrel.



Still need to drill the gas port... The original hole was 0.125".

With the intent of adding the adjustable valve to the gas tube, is there any reason not to just drill out to 0.125" and turn the gas down as necessary?

Also, on drilling the hole, what's the best way to keep from getting burrs inside the bore? Tap a bullet down the bore to "catch" the drill bit as it goes through?

My gas block should be here in a couple days, so hopefully I can get her running by next weekend.

Enjoy the pics and thanks for the advice.

[8D]

[fireball168]

Looks neat!


I've drove a dead soft fishing sinker down the barrel in the past to stop me from drilling too far, then push it back and forth over the gas port to knock the burr off.


I've never been able to find a good drawing or print of the important dimensions on a gas system for a pistol length barrel.

Would you be so kind to post the dimensions from the barrel extension to the gas block shoulder and shoulder to gas port???

[abyssdncr]

Originally Posted by fireball168

Would you be so kind to post the dimensions from the barrel extension to the gas block shoulder and shoulder to gas port???

I took the measurements off of a .223 pistol upper. It was 4.5" from the extension to the shoulder and the port was right about a 0.25" inches off the shoulder. The gas tube was right at 6.5". Don't know if that's what the official "spec" is, but it seems to be working for me on my 7.25" .223 upper. I cut down a carbine length tube to pistol length, so it was cut to fit on this one. I ordered an extra pistol length tube to valve the gas system, so I'll double check with the new tube when it arrives.

My gas block, KNS pins, and flashhider arrived Saturday, so I got the barrel drilled last night and put it all together. Spent the better part of the day loading some Hornady 190s with AA9 to test cycling with subsonics, but then the thunderstorms opened up and couldn't make it to the range. Hopefully, tomorrow evening...

[JFettig]

3.600" from the extension shoulder to the gas block shoulder. .295" from the gas block shoulder to the gas port center.

Jon

[320pf]

Originally Posted by abyssdncr

Still need to drill the gas port... The original hole was 0.125".

With the intent of adding the adjustable valve to the gas tube, is there any reason not to just drill out to 0.125" and turn the gas down as necessary?
[8D]

For a pistol length gas system I would start with a gas port of 0.073". The pressure at the gas port position is much higher than the carbine position. This photo is a plot of gas pressure for a 180 gr super sonic load. The pistol gas port position is located about 4.5" from the breech and the carbine position is about 8" from the breech.



For a sub-sonic load the maximum pressure runs about 25000-40000 psi. So to get an idea of the pressure you can scale the pressure curve in the about diagram. You will probably end up with a gas port of around 0.080 to 0.085. If you get the gas port diameter right you will be able to shoot both sub-sonic and super-sonic loads without an adjustable gas block or tube. This will make your life much easier.

Just make sure you set your depth so you do not drill into the other side of the bore (I am not trying to insult your intelligence just being layer like with the disclaimer )

)

Originally Posted by abyssdncr

Also, on drilling the hole, what's the best way to keep from getting burrs inside the bore? Tap a bullet down the bore to "catch" the drill bit as it goes through?

[8D]

Do not worry about any burrs that you will get drilling the gas port. After you shoot about 10-20 rounds through the barrel the burrs will be gone.... After about 100 round.... burrs??? what burrs?

Good Luck and have some fun

320pf

[abyssdncr]

Originally Posted by 320pf

For a pistol length gas system I would start with a gas port of 0.073".

You will probably end up with a gas port of around 0.080 to 0.085. If you get the gas port diameter right you will be able to shoot both sub-sonic and super-sonic loads without an adjustable gas block or tube. This will make your life much easier.

Just make sure you set your depth so you do not drill into the other side of the bore

320pf

Oops, too late! :D

Just kidding, I was reading that seemed to be where Noveske barrels were ported and that a number of folks ended up around .110" for the 10" barrels. Reckoning pressure vs. dwell - with my barrel being 8" instead of 10"-12" that most folks have, I went ahead and drilled .125".

Almost got the hole perfect - and for the record this is the best use I've ever gotten from a Win 147 FMJ out of this barrel:



So, all dressed up now with nowhere dry to play. Went with the YHM 5C2 and drilled it out 11/32". Hopefully tomorrow will be decent enough to get out to the range and not so windy that my chrony blows over!

[pug]

And my pistol has the shoulder 3.2" from the extension and the port .250" beyond that. Now you see why there aren't many prints around. First it depends on whos tube you use, second whos gas block and third how far you seat the tube into the gas key. The tube needs to extend at least to the center of the cam pin cutout and up to .050" past that. One thing to keep in mind is if you use a used carrier the carbon may have built up in the gas key in relation to where the old tube laid. Trying to force a tube deeper into the carbon buildup can cause some functioning issues.

[pug]

Oops I'm the one whos late. That was meant for Fireball168.

abyssdncr that is one slick rig. Nice pic.

[abyssdncr]

Pug, thanks for the compliment. I've certainly been "bonding" with her the past couple days! :p

Made it to the range this evening and tested a lot of fresh LC brass with Hornady 190 gr. SP and AA9 seated to the cannelure. Loads ranged from 8.1 to 8.6 gr. with 8.4-8.6 yielding SD's of 10 fps. Unfortunately, even with the heaviest charge the bolt doesn't yet lock back properly. However, it did cycle from the magazine and these were no crip loads. Average velocities for the 8.5 and 8.6 loads were 965 fps. I'll give it another try with heavier charges and perhaps some H4227. Groups at 25 yds with iron sights were pretty respectable with the short sight radius and being in somewhat of a rush at less than .75" for 3 or 4 different groups of 5.

Next trip out I'll try to get some glass attached and shoot a bit farther out for a fair shake on group sizes. Meanwhile, progress is progress, and fun is fun!

[Cornholio]

Sounds promising! Thanks for sharing!