BARREL TWIST RATES

[Chrismadrid]

OK - This is something real new to me. But here's my problem.

1) Subsonic
2) Small calibre
3) Pure lead rounds

No hiding - it's an airgun - but a severly overpowered one. It's an itch I just have to scratch.

Her's my dilema - To keep it subsonic I have to use Heavy rounds. And to keep tehm stable I need to use a hollow base it seems - the result is LONG rounds (3 calibres and rising) and I'm getting concerned I may need to alter twist rates.

Standard airgun barrels from Walther are 450mm twist - the option is to use a .17HMR barrel put the jump is HUGE. from 1:16approx to 1:9

Found this "Twist = 150 X D2/L" But seems that speed is a factor and I'm aiming for around 300m/s. The Greenhill is apparently for twice that.

HELP! - If you can.

 

[edge]

IMO, a hollow base conical round should need a relatively slow twist since the CG is most likely pretty far forward. Basically a shuttlecock design, so the nose want to stay in front as opposed to a SMK with the CG in the rear third.

edge.

 

[Chrismadrid]

Ta - Just seen this on Corbins site - so methinks a small investment is called for.

http://www.corbins.com/dc-twist.htm

Quote: DC-TWIST considers the location of the bullet center of gravity, so that with all other parameters equal, a bullet with the weight shifted forward, such as a hollow base wadcutter, an airgun pellet, or a hollow base shotgun slug, would require less spin than a bullet having a long pointed nose and a flat solid base.

Been doing some maths and will try dropping length to 2 calibres.

 

[abinok]

Ive shot 16gr custom .1785" through a standard RWS 34 and had no problems with stabalization. Im not sure what weight youre at with 2-3 calibers or what type of ogive shape/diameter youre using, but thats something to think about.

 

[Chrismadrid]

[ QUOTE ]
Ive shot 16gr custom .1785" through a standard RWS 34 and had no problems with stabalization. Im not sure what weight youre at with 2-3 calibers or what type of ogive shape/diameter youre using, but thats something to think about.

[/ QUOTE ] 3/4E head (will try hemispherical when I get back from the UK).
Depending on the base hollow - 17grains upto 25grains - Power 35ft-lbs ish. 1000fps at 18,6grains.

Also maybe I am being VERY demanding. I'm used to single hole groups at 50m and this wont.

 

[abinok]

Is this a PCP?

Ive played with a gas mainspring 30 caliber, and a 50 caliber that were very high quality, and very well put together by somebody that knew how... neither of them would stack bullets at 50. Back when I was shooting my RWS 100, (olympic format 10M standing) I always wondered if it would... but with the low MV and BC of anything it could shoot, the only way it would ever even remotely happen would be indoors. Most of the best FWB, anschutz, wather... will do .040" ctc at 10M... but even comparing the closest that they do, which would be one of the field target guns from anschutz, they won't stack pellets at 50M either. The above mentioned rifle will do 3/8" at 30yds (indoor), and around an inch at 80(indoor) with the pellets I mentioned. Thats sized for the bore, with a tuner, and a flawless hold, which is about impossible for more than a few shots on a springer.
You may need to re-evaluate your accuracy goal if youre wanting caliber sized groups... or shoot more at a time so that one hole group is bigger /ubbthreads/images/graemlins/grin.gif

 

[Chrismadrid]

Yes PCP. With all the others (we have 7) I get sub 1/2" groups at 50m.

Outdoors at 30m yesterday got bored when my cheap target (the recycle circle on beer tins - "no really dear, I need more targets - <burp&gt") was drilled out in 5 shots and the tin stopped moving - it's an 8mm circle. This with the barrel swapped over into my midrange beast - so it's not the barrel alone.

This with Walther choked barrels and a Spanish unchoked barrel. The others are all sub 20fpe (just). Most 17 and 12 fpe - and 9,5grain pellets.

Walther can do me .17cal in 1:9,1:10 and 1:12 twist - time to save I think.

 

[abinok]

Im assuming youve played with valves and such enough to be sure its not a tune issue? A friends 20cal PCP would shoot practically nothing untill we started spinning dials... then it just started shooting... like it was posessed or something!

 

[Chrismadrid]

YUP - all done. I'm actually factory trained/certified to maintan the guns it's based on.