My B51 thread, convert to .257...or .25

[mackeral5]

 B51's became a topic of discussion last week.  That is a platform that always intrigued me, but I never owned one.  From my vantage point the platform appears to be revered almost as a right of passage for PCP tuners.  Once I realized that I could purchase a NOS unit to my door for a little over $200 I could no longer resist.

So began the research phase....reading threads such as Motorhead's .25 conversion.....the 9mm conversion....Bob Sterne's .22....Ribbonstone' custom stocked B51...fill port issue threads...and so on.

UPS tracking shows it being delivered tomorrow. 

Current plans are to pull the 27" TJ's 1:14 .257 barrel out of my QB .257 project and convert the B51 to .257.  I think the BAM's breech stability will be a significant  improvement over the QB and 40cc or so more air can't hurt either.

If that doesn't work out I have a couple xl725 barrels to choose from   It is getting a .25 conversion one way or another.

More to come....

[WildCJ5]

I had a .25 with a LW barrel. Thing was extremely accurate. I turned it dow to 30 fpe and ended up with a fill pressure under 2,000 psi. They are simple to work on.

I also have a .177 that was my first pcp field target rifle. I have not done any internal work to the valve but have polished everything else I could get my hands on. It is set for 20 fpe and a fill of 2,000.even with the Chinese barrel this rifle is accurate.

 I own a Daystate Huntsman MKII and a LR90, the rifles the BAM is a copy of. They did a good job making the BAM.

The big failure spot on them is the valve stem. The hammer spring from the factory is way stiff and just pounds the valve. A few could removed and the rifles become efficient.

Where did you find them still for sale?

[Ribbonstone]

Think that with minimal tooling,could make a .25 work on a BAM 50/51.

1.They are all old enough now (old stock would be the best you could hope for now) yhat a full-reseal would be advised.
2, Bolt body diameter is cloce to 1/4"....so it's adaptable to .25-.257"without having to make a whole new bolt-body or having to re-machine the breech for a bigger bolt.
3.If you don't want/need a "name brand" barrel,a Hatsan AT44 barrel shank is real close to the tight diameter. ...would need a toch of shortening, but just about factory length.  A little small,but shimming a tiny bit larger is one *(&^ of a lot easier than latheing the shank down.
4. BAM 50/51barrels are glued in place....there aren't any set screws unless you add them

THought about it...have the AT44 .25 barrel,a spare barreled breech,and I know I could make the .25 Hatsan barrel work / fit...and make the bolt work....but have no real urge to convert to .25

[Buldawg76]

following

BD

[Motorhead]

Not sure if you ever had seen this .... A .25 cal conversion done @ 5 years ago with a B50 ?

See: https://www.gatewaytoairguns.org/GTA/index.php?topic=83857.msg804455#msg804455

[Ribbonstone]

Been thinking on it.
HAtsan AT44 (short tube/short barrel) barrel shank is a bit too long.....if you were  to cut it just ahead of the Hatsan barrel port (also removing the internal o-ring),would come out to just about the same length as the3 BAM barrel

DSCN2751 by Robert Dean, on Flickr

Breech-shank is longer than it needs to be...but does no harm other than looking a little goofy (there is room ernough fot rhe added middle diamter to fit.

Receiver inserted shank would be about .007-.008" too SMALL (more like .430" for the HAtsan breech-shank and .437" for the orginal BAM barrel)...but as I'd just glue it back in like the orginal,could shim it on center.

Section for the BAM barrel band would be a little large for the BAM barrel band...like .513"for the hatsan and .500 for the BAM barrel-band....but there looks like enough meat on the upper band to ream/drill/ or polish out the .013".

So it could fit with a cut/shim/polish.....and maybe the bolt could just be adapted with a larger bolt probe o-ring...maybe the bolt would need to be extended.
With that barrel,likely do the conversion with had tools(so long as you had a HAtsan T44 barrel)....files/hacksaw/emeory paper/electric drill.

WORST CASE would be needing a longer bolt body to get a good bolt probe 0-ring seal(but am thinking NOT).



This is really close to the Crosman breech shank diameter....and easier to find a .25 barrel....think the job would be about the same (hand tools)….I just happened to notice the possible use for a HAtsan barrel I already had.

[mackeral5]

Thanks for joining in.   Considering its age I dont expect the gun to hold air.  Based on everything I've read I expect it to be full of aged-hardened grease/oil.  I dont have any expectations of it functioning straight out of the box.  I may put some air in it and shoot a few rounds but I'm not likely to put a scope on it.

The niche this one needs to fill is that of a minimum 100fpe, preferably 125-150fpe slug shooter.  I already have >60fpe pellet shooters in both long bench gun (Gauntlet/AR2078 hybrid) and carbine (QB78 based) versions.  If I'm stuck making this a pellet shooter it may get put on the back burner......

I've read there is enough meat on the receiver to support a .500 barrel, plan is to have the breech opened up to match the donor barrel.   Both potential donors are right at or just under .500 as the xl725 is a unit machined by Baker....

Valve will get a peek poppet, likely with a 2mm stem.  Hammer will get an good bit of weight carved off of it.  I may even see of Rocker1 will make a couple out of MDS once I know what weight range I need.  Porting will be something appropriate for driving 55gr-71gr slugs. 

The info on the bolt is encouraging, on my QB's I've been able to sleeve up .22 bolts with brass tubing and a larger oring, hopefully I can do something similar here. 

More to come, waiting on the big brown teuck...

[mackeral5]

Not sure if you ever had seen this .... A .25 cal conversion done @ 5 years ago with a B50 ?

See: https://www.gatewaytoairguns.org/GTA/index.php?topic=83857.msg804455#msg804455

Not sure if read my post, but your .25 conversion was listed as part of the prerequisite reading. 

[Ribbonstone]

Read it again.... the loading tray section does seem to needs to be just a bit larger to work in .25, but so large that the issue bolt body couldn't work.

Only mentioned the Hatsan barrel becasue I have one....a correct sized barrel would be choice #1, I'm just figuring how to work with what is kicking around the shop.

[Motorhead]

Missed that .. All's well and we got your back ( sort of ) if you pursue this conversion further.

I will state tho ... due to the valves throat being so shallow with transfer hole clearance to sealing surface edge so minimal, your likely going to want an insert seat made so the transfer path can be made much larger that the factory configuration will allow.  IMO

[mackeral5]

More good info, I will definitely study the loading tray/bolt area and the valve throat closely before making any changes.....  thanks again.

[Ribbonstone]

&^^&....now I'm going to have to make the parts I stole from that one to get the other one to run.

[YEMX]

When my lathe is up and running and dialed in, I've been kicking around the idea of making a Cobra valve for my BAM50.  I have one that's been converted to .25 using a GM Mrod barrel.  I'm glad this post has come up!!

[mackeral5]

She arrived safe and sound today.  Couldn't resist putting 2500psi in and putting some shots across the chrony.  Got a few shots of JSB 18gr around 885fps before blowing the probe oring.  Shot cycle is surprisingly crisp for such a heavy hammer and short barrel.  Faux suppressor had to go right out of the box.

It has been a while since i handled a shiny, almost polished,  blued gun, fit and finish is impressive.  Overall the gun handles very well.  My first thumbhole stock, I think I like it.  Bolt is strange, but I expected that.  Trigger is funky shaped, that can be fixed easy enough. 

There isn't as much material in the breech as i expected, so I am having to rethink initial plans.  It is likely I will have to turn down a barrel vs. open up the breech to .500.  Not much room to increase barrel diameter with cf without some form of breech spacer.  This means a 27" barrel will have a lot of skinny, unsupported barrel hanging out the front--unless I make a breech spacer, which isn't very attractive to me at this time. 

More to come. 

[Motorhead]

Yup ... barrel turned to fit receiver. Do keep barrel band out front for support.

[Ribbonstone]

Even if you normally hated barrel bands....that one hangs on the non-pressurized end cap.  So long as it can slide in and out form expansion, you'll be good.

[mackeral5]

Barrel band will definitely remain.  My reference to a lot of long skinny barrel was referring to roughly 9" of barrel beyond the barrel band.  With just 1mm walled cf it isn't going to like a Ronin+extender hanging off of it. 

A lot to ponder.  I said I didn't want to build another short barreled .25, but one thing I did learn from my QB78 builds is to not force something to do/fit where it doesn't quite want to....it has taken a while but I do believe I've finally learned that lesson....

Let's see how I sleep on it tonight.   Maybe clear off a spot on the workbench for it tomorrow.

[wham bam51]

Def following this one. I want to do the same thing however I have discovered that the barrel band on mine actually bends the barrel upward. There is a def wow in the center portion of the barrel. The only way I could see this being possible is if the breech was not machined correctly. Have you removed your barrel band yet? I am def not ok with having a curved barrel but if i am going to be into a huge job to make it right im not sure it's worth it.

I wish you good success with yours.

[Ribbonstone]

Looking over your other post....the band is making the barrel parrallel to the tube.

Could be a bent barrel.
Could be the barrel is straight,but the breech doesn't fit flat to the tube.

[mackeral5]

I loosened my barrel band today.  The barrel band is indeed pulling the barrel down a bit, this is a NIB, unmolested gun. 

At least for the time being it's unmolested, lol....  Haven't dug any deeper, so it could be any of the possibilities--bent barrel...  barrel hole in breech not parallel to breech, breech not seated fully on  tube, breech not machined parallel to tube,  etc....