Extreme 28 Gauge shotgun

[rsterne]

I disagree that the rifling, or lack thereof, has much to do with the velocity.... Very little energy is taken up in imparting spin to the bullet (on the order of 1-2 FPE), and the friction is mostly a matter of fit in the bore.... However, I have found it more difficult to get accurate and consistent Chrony readings with my .410 Hayabusa, loaded with three OOO buck in a shot cup.... However I didn't try taping the shot cup closed, that might make the difference....

I assume, Brent, that you are using a barrel of about 30" or so?.....

Bob

[Extreme Big Bore]

No disrespect intended Bob. I have to disagree on your calculation of 1 to 2 FPE. I have taken bullets and ran them through barrels before and have broken off many hardwood dowel rods trying to get them through the barrel even having to hit the dowel rod with a hammer to get them through I have no way of measuring the FPE to get the bullet through but it is a lot. I have even seen velocity increases by going to a slower twist rate on my 40 caliber I went from a 1 in 14 twist rate to what I have now for the 40 caliber which is a much slower twist rate and saw 50 to 60 FPS different with the same rifle and bullet just different twist rate also I have great accuracy with it. I understand sizing of the bullet is important but even with the perfect sized bullet the rifling still must deform the lead taking FPE away. With the plastic shot cup  I will agree with 1 to 2 FPE to get it down the barrel. One other thing that proves my theory the shotgun will use more air per shot with the same spring as the rifle do to less resistance in the barrel trying to close the valve so a lighter spring is needed. Again I have no scientific proof for any of this but do have the test results and can only speculate why the shotgun performs different than the rifle.

Brent Hoag

[dyotat100]

You said 1000 fps with 1.25 oz. That's 547 gr.

Your saying 80 fps with 435 gr diffence between .72 and 20 gauge. That's a lot more then 80 fps difference shooting 435 gr at 1000 and 547 gr at a 1000.

Yes I know first hand how a faster twist slows the fps down. I have done 2 guns where going from 1:14 to 1:10 was a 40-50 fps loss. This happened in .257 and 7 mm.

How hard would it be to shoot a solid slug or is the choke permanent?

[rsterne]

If you have to hammer a slug through the bore, it's too tight a fit, IMO.... As to twist rate and muzzle velocity, here is what Brian Litz has to say on the subject....

http://bulletin.accurateshooter.com/2015/02/does-barrel-twist-rate-affect-muzzle-velocity-litz-test/

The math says.... " In the case of the .30 caliber 175 grain bullets, the math predicts a loss of 1.25 fps per inch-unit of barrel twist".... and his live fire tests showed more variation shot to shot than from an 8" twist to a 12".... and an actual variation in velocity of 1.33 fps per inch.... (5 fps slower for an 8" twist than a 12")....

Anecdotal evidence is just that, and I would suggest that if you lost a lot of velocity with a faster twist, you need to look at the bullet fit / bore diameter.... I'll prefer to believe the math and the experts, but that's OK, we're allowed to disagree and still get along....

Bob

[Extreme Big Bore]

I used one ounce or 437 grain in both rifle and shotgun for this test and the shotgun will do better than 1000 fps I was down to 4000 psi in my tank and my booster is screwed up. So the old data I have for the 72 cal is based off 4500 and the 437 grain shotgun was based off 4000 psi so it will get faster at 4500 PSI. So I believe we agree a faster twist rate can and does slow the bullet down do to more restriction and it only makes sense that no twist and a plastic shot cup will only increase velocity. As for putting a bullet in this shotgun I really do not want to because it has a poly choke on the end the people at poly choke told me that slugs can be shot through it but it  is hard on the poly choke and this shoots such a nice pattern that I don't want to risk it ultimately this shotgun was to shoot shot of some sort not slugs. I try my best to only put information on here that I believe is accurate and if I find that any information I post on here is not accurate I will be the first to correct it. There is nothing good about anyone whether it be the gun builder or a shooter posting false or inaccurate information.

Brent Hoag

[rsterne]

Brent, what is the barrel length on the 28 ga. and on the .72 cal?....

Bob

[Extreme Big Bore]

Bob the barrel on the 28 gauge with poly choke installed is 28 1/2 inches and the barrel on the 72 I tested was 27 1/2.

[rsterne]

At 28.5", on 4500 psi, your 1214 FPE is right at the limit of what my spreadsheet predicts as a "lofty goal" for a PCP, closer than anything I have seen in the past, so congratulations.... The .72 cal, at 27.5" on 4500 psi would have to reach 2100 FPE to duplicate that relative level of performance, however.... Incidently, these power levels are exactly 50% of the theoretical maximum that can be achieved with that barrel length, bore area, and pressure, regardless of the gas used, just to put things in perspective....

Bob

[Extreme Big Bore]

Sorry about the delay but I will post pictures later this week.
Brent