Big Bore .. or at least Bigger bore BAM 50 conversion project. ( .25 caliber )

[Motorhead]

  this will be an on going thread as project proceeds threw the conversion process 
Will be Doing a Regulated .25 caliber utilizing a M-rod Green Mountain barrel.
It being already sized SAME as the OEM at @ .500" O.D. it should in theory be straight forward in fitting into the BAM receiver.
With the BAM using an @ 1/4" cocking bolt and loading slot already to size feeding a .25 pellet down it should work. ( Hope so anyways )
With the M-rod barrel sealing against probe with an internal o-ring, making a new cocking bolt/probe should also be fairly simple to do.

Anywho just got off the phone with Crosman ordering some parts and that GM barrel too. ( $78.92 ) for the barrel is a screaming bargain weighted against a bare LW or equivalent 

Pictures to follow as we get rollin 

[bubba zanetti]

Good stuff Scott! Looking forward to this build.

BZ

[Rubicon_Joey]

Going to be watching this one closely. Are you building your own regulator?

[Yarp]

I'm taking notes as well.

[Ribbonstone]

This will be interesting…will follow along.

Moving air and making power hasn’t been a problem for BAMs (most of the posts here are about calming them down a bit rather than boosting them up).  Don’t think you’lll have a problem with low power in a .25.

Barrel does have a little step-down for the receiver… down to something like .438” for about 1.5”, but that shouldn’t be insurmountable.

Haven’t had both in my hands at the same time, but memory has the BAM barrel’s transfer port a little ahead of where I’d expect to find it on a Crosman.  If I’m right in how I remember it, then would either  extend the port to end up with an oblong port or rotate the barrel and drill a new one.

[Motorhead]

This will be interesting…will follow along.

Moving air and making power hasn’t been a problem for BAMs (most of the posts here are about calming them down a bit rather than boosting them up).  Don’t think you’lll have a problem with low power in a .25.

Barrel does have a little step-down for the receiver… down to something like .438” for about 1.5”, but that shouldn’t be insurmountable.

Haven’t had both in my hands at the same time, but memory has the BAM barrel’s transfer port a little ahead of where I’d expect to find it on a Crosman.  If I’m right in how I remember it, then would either  extend the port to end up with an oblong port or rotate the barrel and drill a new one.

Transfer port within barrel NOT a nagging issue, worst case JB plug it and rotate 180* and place correctly.
Stepping barrel no issue either, tho given the meat in receiver opt in opening it up in size than barrel down ... we'll see ?
Indeed with the way the BAM valve is manufactured, piece of cake in opening up or even adding a secondary seat if wanted.
As a HUNTER ... single shot will be just fine. 

[Ribbonstone]

All builds have a challenger here and there, but expect you'll figure out the little stuff and have fun doing it.

Do think the BAM valve wants to move air, haven't a clue what it will do in .25 but suspect the increase in bore diameter would let it live up to what seems its potential.

[rsterne]

A .25 cal should be a piece of cake.... I got to 50 FPE in .22 cal. with a good shot string, so you should have no problem in matching that in a regulated .25 cal.... I think .25 cal is about the maximum you can go and get decent size porting without a custom valve.... but IIRC, there is room to take the exhaust port to 3/16" without ruining the valve.... I would expect that if you went unregulated you would be hoping for JSB to release their new 34 gr. pellets, as 70 FPE shouldn't be magic to achieve....  Sounds like a great project....

Bob

[YEMX]

this will be an on going thread as project proceeds threw the conversion process 
Will be Doing a Regulated .25 caliber utilizing a M-rod Green Mountain barrel.
It being already sized SAME as the OEM at @ .500" O.D. it should in theory be straight forward in fitting into the BAM receiver.
With the BAM using an @ 1/4" cocking bolt and loading slot already to size feeding a .25 pellet down it should work. ( Hope so anyways )
With the M-rod barrel sealing against probe with an internal o-ring, making a new cocking bolt/probe should also be fairly simple to do.

Anywho just got off the phone with Crosman ordering some parts and that GM barrel too. ( $78.92 ) for the barrel is a screaming bargain weighted against a bare LW or equivalent 

Pictures to follow as we get rollin 

Strange, I've been kicking around this exact idea for the past two days!!!

[moorepower]

There was a thread on a 9mm B50, but I can no longer find it. It made plenty of power.

[Yarp]

There was a thread on a 9mm B50, but I can no longer find it. It made plenty of power.

Here's a thread on wildman510's build.
     http://www.gatewaytoairguns.org/GTA/index.php?topic=63003.0

The rifle was offered for $400.00 some time back on the "classified" gate.
     http://www.gatewaytoairguns.org/GTA/index.php?topic=73113.0;topicseen#msg697505

[Motorhead]

  Valve as shown was done for the .177 cal tune w/reg that was making 27 ft lbs with 13.43 JSB monsters with a LW nylon hammer at stock stroke, While Cap, Spring & Modified poppet will remain modified as shown, Throat & Transfer size to be opened larger at @ .250" & .210"



An idea on how much lighter I go with poppet spring ( Heavy straight to Conical light )


Poppet valve reduced from @ .420" to @ .310" which closes up a .234" throat ( soon to be .250" )
Stem area within throat very likely to be ground down to @ .090" before were done.


Next we address the known hammer VENT issue where suction created backside of hammer having to vent threw two little holes is altered by 1 large 3/16" hole diagonally drilled in rear cap intersecting the center safety pivot hole just behind the spring seat. * Hole as view from the rear ends up @ 7 o'clock along side trigger block hidden within stock venting into wood void already absent within stock.


Then along comes what I like doing ... LIGHT WEIGHT high speed hammers.  Shown are 4 with 1 being the stock and others lighter nylon ones of altered nose length & effective stroke.  1 also fitted for different spring type.


The spring/s get fitted with rear guides that ALSO vent being it threw there core or along side spring seat.


Addressed next is the PLENUM in which I choose to Maximize the available space by making the spacing via the column method over a bare tube. * This allows Most of the plenum to be at Main tubes I.D. and not the spacer tubes. We gain greater volume in less spaced distance doing it this way. ( Spacing & valve I.D. we're @ 40cc )


Anywho ... where we're at presently 
Scott 

[SpiralGroove]

Hey Motorhead,
Excellent Work -> the legend continues to grow
Kirk

[Motorhead]

Few hours in the shop before the Big race  .... got barrel removed from receiver.
Followed up with making the loading tray area of breech @ .260" along with hole facing barrel.
Once sized and polished Cold Blued to match it up.



Valve apart and throat gets a .250 reamer with transfer passage at @.200" then funnel blended together at throat.


While valve was apart figured a better poppet was in order too. Out came some Peek, 1/8" drill rod to manufacture a reduced stem H.D. valve poppet.



Last for the day a compression fit transfer port also made from Peek playing with OAL until getting a squeeze fit.


Now we wait for the barrel 

[screwwork]

Your build is coming along great Scott!
I hope to meet you at the Pacific Airgun show.

Randy

[rsterne]

How close to the back of the inside of the valve is the 0.200" exhaust port?....

Bob

[Motorhead]

How close to the back of the inside of the valve is the 0.200" exhaust port?....

Bob

As hole dropped into throat it touched off the back edge by a few thousandths. Actually kind of giggled when I did it how idea the placement was keeping the throat shallow, back from the poppet sealing surface sufficiently too.
Any bigger transfer size and you will be in trouble ( IMO )

[SpiralGroove]

Hey Motorhead or Bob,
A couple of quick questions:

1) If poppet end, closing valve remains concave/flat on the end like OEM, IYO, how does tapering the poppet affect its function? http://i113.photobucket.com/albums/n227/kirk_schwarz/Tune12-21-2014008_zps916f7043.jpg

2) I have increased the valve exhaust port to 11/64" on my .22 cal. BAM B51.  This increased hole size left a small arc on the back side of the valve?  I slightly polished stem/throat and valve exhaust port with Mothers Mag polish.  Does this needed to be blended more?  http://i113.photobucket.com/albums/n227/kirk_schwarz/Regulator004_zpsfdd42c9c.jpg

The picture shows back side of valve as in original condition.

Kirk

[rsterne]

Do I understand correctly that the wall thickness between the back of the valve and the front of the exhaust port is only a few thousandths?.... If that is the case, don't be surprised if when you pressurize it, that wall fails and the air blows out the exhaust port.... This may not happen immediately, it could also happen after the poppet has flexed that area a few times, or the reservoir has been filled a few times and it fatigues.... This is not a particularly dangerous situation by itself (although I wouldn't want to have the breech off at the time), unless the gun happens to be loaded with a pellet at the moment of failure, which will result in an unintended firing.... My calculations say that anything under about 0.023" wall thickness is treading on thin ice.... or at least thin aluminum....

Tapering the end of the poppet so that the part that meets the throat is a 45* taper will make the seal surface as small as possible for the pressure.... However, it will tend to wedge the soft poppet material into the hole like a cork in a bottle.... Those effects will tend to cancel each other in terms of reducing the opening force....

Bob

[SpiralGroove]

Hey Bob/Motorhead,
1) Maybe I'm mislabeling things?  I consider where the poppet hits to be the front of the valve.  So in my question, I meant the drill made a slight ( arc in the back (perpendicular) wall (it's thick/deep as it holds the stem) .  Hence, it's no longer a flat donut surface, but one with a small (IoI) vertical groove in it.  I didn't think it would hurt the valve but asked anyway.  Obviously, if the exhaust hole encroaches on the surface where the poppet seals, any blow through would result in a valve that would never hold pressure.

2) Are you saying, eventually, the reduced version of the OEM poppet that Motorhead made (pictured) will prove to require the same force to open as my tapered one, and be more consistent?   

Kirk