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All Springer/NP/PCP Air Gun Discussion General > Big Bore AirGun Gate
.257 Bulldog
mackeral5:
I've had this TJ's 25-20 barrel laying around for some time now and always wanted to try it in one of my Bulldogs. It is a .500 od, 1:14 twist, a little over 27" long. One hurdle in making this happen was the desire for a barrel mounted breech oring. I have become pretty creative in using my very limited fabrication tools and skills, however cutting an internal oring groove is an operation that my hammer and chisels just aren't quite up to task for. Recently a forum member agreed to help me out with this operation, so I am now out of excuses.
The barrel is sleeved with a 11/16 OD thimble of 4130 tubing on the breech end, which mates up to 2 layers of CF tubing--a 15x13 then a 15x17. The transition from 4130 to CF occurs under the rear shroud adapter. The 4130 and CF tubing is bonded with Loctite 680.
The .257 barrel will actually end up as a swappable complete "upper" with the barrel/shroud semi-permanently attached, there is no reason to separate the 2. Here is a picture of my .357 upper and a possible shroud length. I will be installing a 17mm slip on 1/2-20 adapter once it arrives from Poland. The shroud will end up somewhere between the 2 shown. A second rear shroud adapter will be mounted at the front of the shroud, with side screws added to make the barrel/shroud one rigid unit.
The bulldog's design makes caliber swaps challenging, if the caliber swap system requires interchangeable bolt probes. To remove the OEM bulldog bolt, it requires almost complete disassembly, at minimum some of the trigger sear/pins must be removed to access the cocking lug/bolt. Long term, I hope to have a bolt system made very similar to the WAR style which will allow for simple caliber swaps.
I've read a lot of posts from Marco over the years where he gives advise along the lines of "build the powerplant for a .357 and put a .257 barrel on top of it." That is what we have here. The bottom end is ported .310 with full bore porting in the .257 barrel. A bolt probe is in the way, but I have tried to minimize it's size.
I've also read where it takes a lot of plenum and pressure to get .257 bullets up to speed. This experiment has taught me that there are no exceptions to physics..... Initially I tested with a regulated tube and the Huma set around 125bar. 71 gr slugs would not break 900fps, using approximately 120cc of plenum at this reg setting. I didn't invest much time here, and I may revisit this but for now I will ditch the reg.
Next I put on an unregulated tube and found a twin spring combination that worked pretty well.
Filling to 3000psi, and dialing up HST until it will just barely cock---
Hunters Supply 87gr 930fps 167fpe
Hunters Supply 71gr 1030fps 167fpe
There may be a little more in it with hammer weight, or spring, but right now I don't really see any need to push it harder.
Dialing HST back a bit gives a 4 shot string 982, 983, 984, 972 fps with Hunters Supply 71gr. Call it 4 shots of 150fpe.
I'm not quite sure where this one will end up, but I like the data I see so far. For example, when using this same barrel on my QB .257 build with a powerhouse valve, it maxed out with the 87gr at 935fps, but it took 3500psi to do so whereas the Bulldog is getting 930fps on 3000psi. I am assuming this is largely due to the Bulldog having almost double the air storage of the QB .257 build, thus maintaining a much higher average pressure during the shot. I also note the barrel was cut down approximately 1/2" when converting it fitting it to the Bulldog.
Once I get an LDC mounted I will put some lead on target. That may be a week or two while waiting on the adapter to arrive from Poland.
redlined_b16a:
I was brewing up a plan in my head last week for a .257 or .308 barrel on the Bd platform.
Outstanding work!
Thinking a longer .357 barrel could be used as well.
I did come across a .357 1:16 twist 34" long blank.
But I have no clue where to start to make the lead in.
Barrel blank was a 1" o.d.
mackeral5:
--- Quote from: redlined_b16a on December 18, 2020, 01:26:04 PM ---I was brewing up a plan in my head last week for a .257 or .308 barrel on the Bd platform.
Outstanding work!
Thinking a longer .357 barrel could be used as well.
I did come across a .357 1:16 twist 34" long blank.
But I have no clue where to start to make the lead in.
Barrel blank was a 1" o.d.
--- End quote ---
Thanks for the compliment. The chamber/lead is one of the easier parts, IMO..... search for posts about a taper pin reamer. Regarding barrel length---I wish I would have had 30+" of barrel, but all I had was a little over 27"....
FuzzyGrub:
Nice work, Mike. Thx for posting. Cal changes on BD's looked very challenging to me. Glad you tackled it and provided the info for everyone. :)
Look forward to seeing that bolt set-up.
rsterne:
Impressive.... It is always easier to go smaller on caliber to get big (relative) power than the other way....
Bob
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