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< 20fpe BC comparison

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My theory for how barrel affect BC of a given ammo is as below:

1. Less pellet impressions(less in numbers and depth) made by lands the better. We all know that smoother the surface the better for aerodynamic, give how light the pellets are any unevenness will affect the overall BC
2. Smoother the lands impressions (rounded vs sharp edges) are the better, same reason as point 1
3. Slower the barrel twist the better the BC. As long as the pellet is stabilized (which doesn’t require spin at all) any more twist is not needed. But faster the twist rate the faster the pellet will spin or faster the edge of the pellet will travel through air. As the impression from lands move/spin/cut through air faster due to faster twist rate it increases the drag exponentially because that’s how aerodynamic works(non-linear equation).

So my conclusions from my hypothesis is slow twist poly barrel would be best from BC perspective for pellets. my FT gun has a slow twist poly and its trajectory matches 10.3’s BC of 0.029 in ballistic calculation dead nut on. However I’m not sure if that’s better than what a LW 1:17.7 twist rate barrel would do. I shoot WFTF or 11.6 FPE so maybe at that speed the BC difference won’t show up as much but it certainly has noticeably less wind drift compared to others shooting 8.44 from tradition lands and groove barrels.m which I know is an apple to orange comparison.


HOWEVER, even if my hypothesis is true, sourcing a slow twist poly barrel is a whole different challenge/conversation. Just ask @Motorhead.
 
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qball said:
My theory for how barrel affect BC of a given ammo is as below:

1. Less pellet impressions(less in numbers and depth) made by lands the better. We all know that smoother the surface the better for aerodynamic, give how light the pellets are any unevenness will affect the overall BC
2. Smoother the lands impressions (rounded vs sharp edges) are the better, same reason as point 1
3. Slower the barrel twist the better the BC. As long as the pellet is stabilized (which doesn’t require spin at all) any more twist is not needed. But faster the twist rate the faster the pellet will spin or faster the edge of the pellet will travel through air. As the impression from lands move/spin/cut through air faster due to faster twist rate it increases the drag exponentially because that’s how aerodynamic works(non-linear equation).

So my conclusions from my hypothesis is slow twist poly barrel would be best from BC perspective for pellets. my FT gun has a slow twist poly and its trajectory matches 10.3’s BC of 0.029 in ballistic calculation dead nut on. However I’m not sure if that’s better than what a LW 1:17.7 twist rate barrel would do. I shoot WFTF or 11.6 FPE so maybe at that speed the BC difference won’t show up as much but it certainly has noticeably less wind drift compared to others shooting 8.44 from tradition lands and groove barrels.m which I know is an apple to orange comparison.


HOWEVER, even if my hypothesis is true, sourcing a slow twist poly barrel is a whole different challenge/conversation. Just ask @Motorhead.
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LW does sell Slow Twist .177 poly blanks .... now if or not there the same spec as whats in your Thomas ? .. that i have no idea.

Stainless: https://www.lothar-walther.com/1515/airgun-blank-stainless-choked-twist-36.177-air-polygon-od.63-l-23.82?c=273


Standard: https://www.lothar-walther.com/gun-barrel-blanks/airgun-barrels/polygon-profile-with-choke/1569/airgun-blank-choked-twist-36.177-air-polygon-od.63-l-23.82-precision-rifle-steel-tube?c=273
 
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Tim McMurray has the 1:36 LW polys, at times. About 8 months ago I ordered one for a friend to drop into his USFT. Had it less than a week later. That quick might be an anomaly, but that's how it worked out.

Edit: Dan reminded me that there were multiple unanswered emails in the mix. Only after we worked though a mutual friend (Chris) that lives somewhat near Tim and shoots with him did we get anywhere. Had the slow twist in-hand within a week of Chris letting Tim know we wanted one.
 
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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.

Scott S... two rifles on the chart have HW's, purchased from Tim at the end of the Simple Simon project but before the USFT production. They are both fairly high bc's... Theoben, 0.0348, and BlueBaby , 0.0367. An interesting difference in those two is the way air is delivered , which I commented on in the original thread. Since the air pressure would build more slowly in the ProTarget or USFT, it has me wondering about obturation as a bc component but I've not come up with or taken the time to pursue a good way to capture pellets undamaged . I also agree about the symmetryand groove depth but we have not much control over that without ordering very expensive custom barrels.

Wayne... you sold it to me because it was a 249 barrel, not 250 😉 It's not a slow twist so must be 1 in 17.7.

Interesting stuff !!
Bob
 
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Arzrover said:
Lots of good thoughts here... thanks for contributing.
Click to expand...

Arzrover said:
Scott S... it has me wondering about obturation as a bc component but I've not come up with or taken the time to pursue a good way to capture pellets undamaged . I also agree about the symmetryand groove depth but we have not much control over that without ordering very expensive custom barrels.

Interesting stuff !!
Bob
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If we wish to add some loose data info:

The HW .20 is fitted to a RAW TM-1000
In doing my barrel prep it was set up having NO Freebore or Leade. Pellet upon placing in barrel is into full rifling contact. As probe contact pellet taking it deep to just past transfer port. Loaded this way the pellet is fully in rifling and only waiting for a light push to move it further on down the barrel.
*Pellets breakaway friction is VERY minimal set as such.
Rifle has my own valve, a very light hammer sprung rather stiff via SSG. Firing cycle is extremely fast and short dwell.

I'm guessing the skirt getting deformed ( obturation ) is very minimal which was my intent when setting up this particular rifle.
The set up has proven itself over & over threw the years to be very accurate as well consistent.

FYI
 
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Motorhead said:
Doubtful if said barrel/s are of good quality and have a good crown and symmetrical leade.
Always have found the barrel itself as to the Land / Groove diameter differential playing the bigger part.
Tall lands disrupt the pellets outer most diameter head & skirt more so than shorter height lands .. similar to a poly barrel.
* In other words: have a smaller differential to land / groove diameters.
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The Land/Groove diameter differential's impact is a very interesting insight. What is the typical differential in a LW barrel? What is the differential in your HW? How do you measure this? I ask because I wonder if a LW standard barrel could have its lands ground down and thereby reduce obturation and improve BC. Has anyone tried this? Could it turn a .177 into a .20?
 
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Understand as with ALL machining specifications there is a +/- tolerance allowed.
Now what that is ? being factory only knowledge.
Evidence of it being so is clearly shown if one purchases a common barrel over & over threw out a few seasons or more. Clearly the sizing, fit and finish of bore changes from new tooling to end of life tooling. Again we never know what we are getting ?
 
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Arzrover said:
CC, that order of bc is exactly how it tested for me when we did the 25 cal ART project.
The 177 has been a little different... as shown in our chart...
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Interesting... I think the FX ST barrel he used was one of the old ones that wasn't rifled the entire length like the new(er) STX ones. I'd think the STX would be similar to the poly, since looking down the bore it appears to be poly. Just a guess.
I'd really like to shoot the JSB 10.34 from my standard Pulsar, but the 10.65 H&N are just more accurate in my gun - unfortunately the BC isn't as good. :(
 
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My lab coat doesn't fit anymore so I had to get a visitor's pass at the front desk to jump into this conversation.
What is the brain trust's opinion about sub 12 fpe? Would the slow poly LW barrel have enough impact at lower energy levels to be worth a change, or is the 1:17 poly a better choice for 8-9 gr pellets? I realize this will be gun, pellet and barrel dependent, but I'm wondering if there is a general trend that anyone has noticed when it comes to poly barrels at 12 fpe. Maybe I'll take a look at the UK forums and see what they have to say on the subject.
 
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Well, skimming the UK forums may not have been the best idea, but I think I gleaned a little bit of usable info. Seems like the thinking is that slower twist rate is good for full power but less ideal for 12 fpe. I think the weight of the pellet comes into play as well, since a lot of them are playing with slow twist unchoked barrels for shooting slugs at 12 fpe.
I would still like to hear from any of you that may have thoughts on twist rate as it applies to 12 fpe. I think that due to our freedom from power restrictions, we might have different first hand knowledge (such as being able to test the same barrel and pellet setup with different power levels).
I'm going to take some Advil and lie down for a bit.
 
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scotton said:
Well, skimming the UK forums may not have been the best idea, but I think I gleaned a little bit of usable info. Seems like the thinking is that slower twist rate is good for full power but less ideal for 12 fpe. I think the weight of the pellet comes into play as well, since a lot of them are playing with slow twist unchoked barrels for shooting slugs at 12 fpe.
I would still like to hear from any of you that may have thoughts on twist rate as it applies to 12 fpe. I think that due to our freedom from power restrictions, we might have different first hand knowledge (such as being able to test the same barrel and pellet setup with different power levels).
I'm going to take some Advil and lie down for a bit.
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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 ?
 
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I agree with Scott's commentary but have not shot a LOT at 12 ft lb except springers.

Just wanted to point out the RN10 on the chart. Not sure who's barrel it is but with 8.4s at 14 ft lb, the bc was 0.0336. It's incredibly good in the wind... better than a lot of my 20 ft lb rifles, even some of the 20 ft lb poly's. It was built sometime around 1995. I'm pretty sure it will be close to that at 12 ft lb. The rifling is very fine and the bore very even.
I also rebarreled a Steyr for a friend with an original 10M hammer forged Austrian made one from about 2002. It was truly incredible ! I posted about it and shoed a sub moa group at 100 yds in still conditions... at 12 ft lb. The first 3 were literally overlapping but didn't hold enough for the slight breeze change that made it just under moa. The bore looked like glass in my bore scope .

Scott's earlier comments about symmetry and rifling depth are likely the major reasons then add the possibility of low obturation because of the valve porting arrangement . Still have to experiment with that.
Bob
 
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My only data is just shooting groups in the wind with different pellets.. When I was shooting WFTF class with my USFT at 12fpe with a LW slow poly barrel. I found the 10.34 JSB & Air Arms really out preformed the 8.4gr and only hit a little lower than the 8.4 at 55 yards.. so that's what I was shooting the last two years of my WFTF class shooting. I don't know the BC at 12fp for the 10.34s but it has to be pretty good compared with the 8.4s if one compares the point of impact at 55 yards.

Take note of what the top WFTF shooters are using... a good number of them ARE shooting the 10.34s at 700pfs to 715fps.

Wayne
 
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