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

[Ribbonstone]

Different post...but worth a look-see.

https://www.gatewaytoairguns.org/GTA/index.php?topic=173800.0

[mackeral5]

Different post...but worth a look-see.

https://www.gatewaytoairguns.org/GTA/index.php?topic=173800.0

Thanks!

[mackeral5]

I went ahead and disassembled.  I am pretty close to accepting that most everything I said earlier in this thread was a bunch of jibberish, stated from a very misinformed position, based on several inaccurate assumptions.

At maximum, I may rebarrel as a .25 pellet shooter.  .257 is out of the question.  Too many modifications will be required, especially in the bolt area, for what I would consider a safe conversion.  It's kind of ironic that I abandoned the QB platform as my .257 and thought I could move to the B51.  I cited breech instability as the primary reason.  The B51 solves breech instability, in a very robust way with 4 point breech retention, and room to increase size of fasteners if needed.   However the B51 introduces a problem the QB didn't have---a lack of robust bolt retention.  My QB .257 had a single 5/16 grade 8 bolt handle.  There's virtually no method of upgrading the B51's bolt retention without significant modification/redesign.  So the B51 solved one problem and presented another. 

I am still very pleased with the value in the B51.  I just need a little more time to determine what I'm going to do with it long term.  I'm pretty sure whatever direction will be <60fpe. 

Options could be:
A regulated .25 JSB 25gr shooter 
A regulated .22 JSB Monster Redesigned shooter. 

For now, I'm going to get to know the platform as an unregulated .22 JSB 18.13gr shooter.  Peek poppet, light hammer, hammer free flight are the first mods.  Will likely not touch porting initially. 

Here is a pic showing the shallow throat previously referenced by Motorhead.  i haven't seen this pic in other threads so I did want to share.  The valve exhaust measures around .110, there's not much room in there. There is plenty of room to increase size rearward of the exhaust port, if one wanted to do so.  For my purposes I don't believe I will have to go to such extremes....

[nervoustrigger]

Hi Mike, for others reading along, I just want to point out the concern you raised about the bolt has less to do with what happens in a normal firing cycle than it does if the bolt were accidentally left open.  Let's just say we're operating at full 3000psi pressure in .257 cal.  The bolt sees at most a force of 172lbs or so.  The cross piece (shear pin) can be made to easily withstand that.  However if the bolt were accidentally left unlatched, the pressure would send it accelerating backward until it crashes into something.  That impact has the potential to shear off the cross piece and send the bolt back at the operator.
 
The how fast and how far are the relevant considerations.  The "how fast" would depend on how soon the mass of the pellet accelerates in the forward direction versus the relatively much more massive bolt the rearward direction.  That would influence the average pressure acting upon the bolt.  As soon as the bolt has moved enough for the O-ring to clear the breech opening (not far at all), this pressure spills into the ether.  The "how far" will depend on the location of the hammer because that's what the cross piece is going to slap into with whatever energy and momentum were imparted to it before the O-ring cleared the breech opening.
 
Going through this mental exercise, I'm less concerned now than when I started.  But I still would be inclined to test this abnormal condition a few times to be sure before I went sticking my collarbone behind the bolt handle.

[PikeP]

FWIW, an unlatched bolt has to first compress the spring fully and re-engage the sear prior to being sent further back, and from there provided the hammer hits the rear assembly opposed to the cocking lug, then your rear plug would be withstanding the force left over after compressing your spring...which is fine provided...

You attach the rear plug to the tube/frame with 3 fasteners... you shouldn't have any bolt breaking and being sent into your face any further than it can cock...so redesigning this bolt to be cocked from the side of the receiver opposed to the rear with the above preventative measures, you would have zero concern in an unlatched condition, your only concern needs to be how robust the locking bolt is in the locked position...

Honestly I wouldn't want any big bore to be full rear cocking for the above mentioned reasons, any unlocked bolt is gonna come flying back in the direction you cock the gun...how far back is based on design...and a big bore that cocks on the opposite side that your cheek rests is probably the safest. This also adds an extra stop for a cocking lug breaking in the locked position, the bolt would be contained within the receiver upon breakage as opposed to having the freedom to exit the rear of the breech.

Additionally, in a big bore you should design the bolt / cocking mechanism to have its lug be much stronger than the one in the hammer, so if one were to break, it would be the hammers...

The above build would have 3 fail safes...side of the receiver, hammer lug weaker than bolt lug for a 'controlled' failure, and ensuring that the cocking lug isn't what stops the bolt in an unlatched condition.

[mackeral5]

Great info, thanks for chiming in. 

When I entered into the mental planning phases of this project i had not given enough (not any) attention the bolt arrangement of the B51.  No worries though, I am fond enough of the guns design to not worry about abandoning the .257 efforts....  Will probably put the .257 barrel in one of my Bulldogs.  I can do that complete job except modify the bolt probe in house.....

Here are a couple of pictures of the B51 bolt and cocking lug, with a JSB 18.13 alongside for comparison.  Just my personal opinion, but there's not enough meat here for me to work with.  My tools/methods lack the precision needed to work on such small pieces with any level of confidence. 

So, initially an unregulated .22 pellet shooter, but it may later become a .25 pellet shooter.....

[nervoustrigger]

Pike, good point about the rearward force having to compress the hammer spring.  I was picturing the OEM hammer which is massive and heavy, and I suspect the comparatively lightweight bolt traveling at high velocity would hit it like a solid wall.  If that impact snaps off the cocking lug, gone is the need to compress the hammer spring by any meaningful amount.  However Mike did indicate his plan to make a lightweight hammer so it's probably fair to say it would be a situation lying somewhere between the two extremes.

[mackeral5]

Pike, good point about the rearward force having to compress the hammer spring.  I was picturing the OEM hammer which is massive and heavy, and I suspect the comparatively lightweight bolt traveling at high velocity would hit it like a solid wall.  If that impact snaps off the cocking lug, gone is the need to compress the hammer spring by any meaningful amount.  However Mike did indicate his plan to make a lightweight hammer so it's probably fair to say it would be a situation lying somewhere between the two extremes.

On the B50/51, the cocking lug and bolt retention lug are the same piece.  Once it is removed/broken, there is nothing else left to retain the bolt. 

I wasn't stressed so much about the safety aspect of the bolt flying out, just that I didn't feel comfortable with it being a good fit for long term use in a >100fpe application---repeated cocking against however much hammer spring would be required, forces when firing, etc.  There's not much meat around the threaded hole in the bolt, I could see myself encountering some future difficulties in this area.   I likely confused matters when i used the word "safe".  I would have been better served replacing "safe" with "robust" or "reliable".

[Ribbonstone]

Maybe from a safety point of view,would be better to enlarge the bolt pathway from the front (lets say .262"),and make a two diameter bolt....long enough large diamer to operate back and  cock,but too large to pass the standard diameter back even if the locking/cocking piece was missing?

[YEMX]

Maybe a separate hammer knob?  Separate the bolt from the hammer...  Make them independent of each other- no worries about the bolt lug shearing?  Although, you'd still be dealing with the force of firing... 

When I rebuild my B50, that's what I plan on doing- a hammer knob.

[mackeral5]

Things are progressing along....  Initially this will be a Cobra-valved, regulated .22.  One of the Lithuanian regulators is in route, hopefully it will arrive in a couple of weeks.  A light hammer from Rocker1 is scheduled to arrive today.

OEM Barrel has been lapped and polished. 

Tube deburred.

Peek poppet with OEM stem diameter has been made.   OEM seat modified to seal on the inner edge, yielding the smallest diameter seal possible.  Valve currently installed in pressure tube for leak testing.  Once leak test is successful i will disassemble and cut the oring groove for the cobra chamber. 

Porting is close to bore size. 

How the gun performs with the factory barrel will dictate next steps.  if my B51 is as capable as my QB78 documented here https://www.gatewaytoairguns.org/GTA/index.php?topic=166800.0  it will remain a regulated .22.  If not then it will get rebarreled to .25 and be set up for JSB 25gr around 45fpe.  The powerplant is being built so that it will work well in .25 as is, but if I go .25 I will likely slim the valve stem down to 2mm, increase porting by 10-20 thousandths, and be content with somewhere in the neighborhood of 80% bore porting.  This will basically duplicate my P15 from a powerplant perspective, but with a much shorter transfer path.....

Pics to come.  Remember though, my tooling and methods are very crude so these parts should be judged merely by functionality....

[mackeral5]

My shipment from Rocker1 showed up.  I asked David for as light of a B51 hammer as he could make with a peek striker.  This piece of artwork is what I received.



25 grams.  Plenty of room to go lighter if I want.  I'm still grinning...

David always delivers!

[nervoustrigger]

Very nice!
 
Just a quick thinking out loud moment.  If memory serves, the stroke length on the B50 is not particularly long so I don't know that you'll want a head on the PEEK striker standing proud and reducing the stroke further.  It may work great with the benefit of a new PEEK poppet at a reduced sealing diameter but I just wanted to mention it.  And of course it's simple to address if it were to happen so it's not like you need to make special preparations or anything like that.
 
Also, I couldn't tell from the photo perspective but if there isn't a hole or two drilled axially through the hammer to allow air to move between the forward face and rear face, I'd say go ahead and do that.  Otherwise a suction gets created behind it as it flings forward, introducing a variable hammer strike.  Some folks dealt with it on the OEM hammer by just removing the safety assembly from the back of the tube to allow air exchange to the outside world.

[Motorhead]

I would AGREE ... with a Light Hammer your going to want / need longer stroke and what I would suggest is the adjustable striker being SHORTER than OEM specs able to be adjusted out to equal OEM length if needed.

Another issue your going to have is COCKING FORCE being MORE SPRING is going to be needed being this is a HIGH POWER conversion.
The threaded in cocking lug on bolt is NOT going to be happy and has a high likelihood to shear off when cocked.


FYI .....

[mackeral5]

Great points.  I literally just finished cutting a groove down each side of the hammer for rear-venting. 

Regarding hammer travel----  1)  The striker is fully adjustable to the point of being sunk into the nose of the hammer.  This may or may not be enough, if so there is room to move it further back.  Also note the valve stem is only going to protrude .15" or so from the rear of the valve.  Hammer travel should be equal to or greater than what I am used to working with in my QB's.... 

Hammer weights on my QB78 conversions (pressures ranging from regulated at 117 bar to 3kpsi unregulated) such as the one previously mentioned are typically 14-18 grams including the cocking pin.  Valve throats are similar.  The Rocker1 B51 hammer weighs in just over 26 grams without the guide screw.

I anticipate a reg setting no greater than 125bar, will likely end up a little under 120bar.  Targeting 20-25cc total plenum to allow for the switch to .25 if I am unhappy with the .22. 

Based on these things, I do not expect to require excessive cocking force.  If so, I can either increase stroke or add hammer weight.

Thanks for the watch-outs, they are indeed helpful.  Know that the general specs I am using here were developed while researching posts each of you have made over the years, coupled with some of my own testing.

[Ribbonstone]

Cocking force does become and issue....it's a straight pull back,every pound of spring tension is noticable.....noting will change that foorce...two finger pull makes it noricable....bolt handle spreads the load.....side levers get a lever advantage....but BAM's basically are like pulling a slight shot or bow string.

Yeah...know they are a "kind of"  clone of an old  Daystate....on the Day State reissue, it's a bolt handle.

BUT regulated,you aren't getting it to run at 200BAR....are getting it to run at regulator out put....maybe 100-125BAR?  That's going to help alot.

[Motorhead]

Missed the point after talking 257 that it was to be a REGULATED .22 cal.

Yea you should be alright 

[mackeral5]

Missed the point after talking 257 that it was to be a REGULATED .22 cal.

Yea you should be alright 

Got it, Thank you, I wish we could edit old post titles, that would help.  Somewhere between starting this thread and now I came to my senses regarding goals/direction of this build. 

[mackeral5]

Finished the valve work and have everything sealed up.  Starting point on the cobra vent is .065.  Vent size seems somewhat related to chamber diameter, this chamber is approximately 5/16 ID. 

The Cobra chamber occupies the OEM air passage so alternate path(s) had to be created.  This was accomplished by drilling a few more holes in the valve cap.  I don't own a drill press so the alternate air paths are not very pretty, but they fully accomplish their intended goal. 

I didn't take a pic, but I bored and turned down a piece of 3/8 delrin rod to make the transfer port.





I picked a short stiff spring from my inventory as a starting point.  Here is the OEM spring and the one I'll be using for initial testing. 



This spring results in the tiniest amount of preload against the valve stem with the striker screwed in flush, yielded the longest hammer travel.  Note poppet stem protrusion has not been fine tuned, it stands proud approximately .200", it needs to have about .050" removed.  that will come once i tear down to install the regulator. 

Using this valve and striker system yielded a starting velocity of 800fps at 2000psi and a peak of 945fps at 1400psi using JSB 18.13gr.  Further down the slope 885fps was achieved with a little over 1000psi. 

As expected with large porting and an unregulated Cobra valve, ES is EXTREME to say the least. 

The cobra chamber venting is a little tight, even at lower pressure and 885fps, with the hammer being way overdriven, the shot cycle was still a sharp SNAP.  Once the regulator is in place I will tweak the vent. 

I just realized that I forgot to tighten the valve cap after assembly, but it is perfectly holding air.  I have experienced this with my QB's as well..... 

[mackeral5]

It's been a while since i posted on this one.  I settled for a plenum of approximately 25-30cc and vented the tube.  Should I decide to follow the original plan and convert to .25 it shouldn't be too challenging to achieve 45fpe... 

Right now it is tuned for upwards of 40 shots JSB 18gr at 905fps.  I need to pull the reg and drop it a bit so I can dial velocity back to 885-890fps.  It is using a twin spring setup with the hammer in free flight.  Cocking effort is nice and easy.  The shot cycle is a nice snappy POP thanks to the light Rocker1 mds hammer and peek-poppet Cobra valve combination.  Next week an LDC from Rocker 1 should arrive.

A note about the well documented need to vent the hammer/tube....  I previously added a couple of shallow v shaped slots down each side of the hammer, thinking this would be sufficient for venting the hammer.  For all testing I did not have the safety installed, so there was an additional vent in the form of the safety screw hole.  Last night I was finally to the point where I was "finished" enough to put the safety back in place.  This morning I put a few pellets on target at 50 yards.  Point of impact was very low...  I went inside an put a few pellets over the chronograph---775, 780, etc.  Then it hit me, the only thing that had changed was I installed the safety.....which in turn meant the hammer no longer had a supplemental vent.  I again removed the safety, velocity restored to the 900fps range. 

I ended up venting the rear cap with a 1/4" hole drilled at roughly a 45degree angle, creating a vent into the inletted area below the end cap.  Then after reinstalling the safety velocity was in the 900fps range again.  Until I remove the reg to make a slight adjustment down, as a means of improving ES I added a shim to the hammer springs, increasing velocity to 905fps

So far my journey with the B51 has been a good experience.  I really wish I would have started with the B50/51 platform sooner.  It certainly would have been much cheaper than building QB78-based PCP's, lol...