< 20fpe BC comparison

[qball]

Have been told by some in the manufacturing business of high end AG's .... who use slow twist poly .177's @ 36-1 twist, that they do not work well and not recommended for 12fpe rifles. Having personally tried it, customers & friends try it .. IT DOES NOT WORK WELL & accuracy is sub par.

IIRC, reading posts from a few years ago, manufacturers out of europe heavy in 12fpe take twist down to & 16-1 to 18-1 ranges for the @ 8 grain 12fpe uses.

You will start learning that rules of gyroscopic stability at SUBSONIC speeds in many applications don't follow the observed & practiced rules of supersonic projectiles. We have a few ballistic gurus among us that could explain better why that is ?

Well, that explains my sub par performance. That’s my excuse and I’m sticking to it!

 

[Motorhead]

Or corrected in that statement ... slow twist poly .177 and the 8.4 pellet.
The ST barrel and 10.3's at >12fpe seem to do quite well.

 

[Scotchmo]

Lots of good thoughts here... thanks for contributing.
Scott H... the slow twist poly only comes in 16 mm so it will need to be reduced for the DW which opens the bore a little. Not sure how that will affect it.
…
Bob

A 16mm barrel just about fits (tight). I prepped a LW 16mm stainless slow twist poly for my Delta Wolf. The mild steel LW barrels run slightly over 16mm diameter. The stainless LW barrels that I got were almost exactly 16mm diameter. I sanded down the back half of the barrel diameter just slightly to about 15.95mm for the length that now more easily slips into the receiver. And maybe 15.9mm just before it steps down to the breech end which ended up about 13.4mm diameter.

I removed the o-ring to allow the nearly 16mm barrel to slide all the way in. In place of the Delta Wolf picatinny block, I used a cast prototype block with a built in 16mm barrel clamp.

Quite a few hours of careful work spread out over a few days.

I did the first test today with a direct comparison to my Skyhawk which has a normal twist poly. The Skyhawk at first appears to have a slight accuracy edge over the slow twist in the Delta Wolf. No final judgement though as I’m still much more familiar/comfortable with the Skyhawk, which I’ve been shooting for a few years. All shooting was done from a seated FT position.

As far as BC, the slow twist was better than the regular twist poly. I still need to do some velocity degradation tests, but based on drop, this is what BCs I used to get the trajectories to match (from zero out to 55yds):

All .177 caliber.

Delta Wolf with 17” factory barrel. LW choked 1:17.7 standard? 10.3gr@930fps:
GA BC = 0.029-0.030

Skyhawk with 23.75” LW choked 1:17.7 poly. 10.3gr@930fps:
GA BC = 0.033-0.034

Delta Wolf with 23.75” LW SS choked 1:36 poly. 10.3gr@920fps:
GA BC = 0.037-0.039

Mostly agrees with your data.

It was windy. Hoping to test in calmer conditions soon. I have a Labradar, which I currently use with my slug guns. I plan on using it to get better BC data for my <20fpe airguns, but I need to get a microphone trigger for these lower FPE pellets.

 

[Franklink]

@Scotchmo

Thanks for adding another data point. I'm not sure how I missed it til today.

Yes I agree, what you're seeing seems to support the general take-away from Bobbies testing sessions (and from many of our anecdotal experiences shooting matches) that 12 land and groove < standard twist rate polygonal < slow twist polygonal when comparing BCs produced in the sub 20fpe realm of power output.

I might try to find some time to repeat the comparison that you just did as I have access to the same 3 rifling profiles in .177 as your above example (a 17" 12 land and groove from Ghost Carbine, a 23"standard twist rate polygonal in a Veteran, and a 23" slow twist poly in a USFT). I've measured BCs from the USFT only. I have compared 55 yard wind deflection from the USFT vs Veteran (slow twist poly versus standard twist rate poly) and the slow twist had less deflection, but I can't remember measuring BCs from that standard twist poly. And I know for sure that I haven't measured BCs from the 17" 12 land and groove that came in the Ghost Carbine.

All very interesting information.

(as a side note.....am I understanding correctly that you kept most of the 16mm od for the barrel you machined for the Delta, but omitted the orings that center the barrel in the chassis, instead relying on your aftermarket casting for barrel fixing?)

 

[Scotchmo]

@Scotchmo
…

(as a side note.....am I understanding correctly that you kept most of the 16mm od for the barrel you machined for the Delta, but omitted the orings that center the barrel in the chassis, instead relying on your aftermarket casting for barrel fixing?)

Yes. That’s what I did. With all the easy swapping of barrels, I like the concept, and it is convenient for testing various configurations, but in the end, I like to stick with the best of them and don’t want to swap. The Delta Wolf is OK in a sub 20fpe configuration, but the platform is overkill and is better suited for the 50fpe+ range. I decided it is going back to .25 caliber and staying that way for the main purpose that I got it (EBR/EFT/maybe PRS).

I already have a great shooting 20fpe bullpup (SPA/Diana P15) configured just the way I like for AAFTA FT. It’s got an accurate 24” LW 1:17.7 poly barrel. My FT shooting position does well with a very light airgun, and the P15 is about the lightest available. After considerable side by side comparison, I see no advantage for me to switch to the Delta Wolf in sub 20FPE FT.