BAM B-51 - Better Late Than Never

[rsterne]

This afternoon I made an RVA (rear velocity adjuster / power adjuster / preload adjuster) and shortened the spring guide to compensate for it.... Here are the parts....



On the left is the guide, shortened 1/4".... In the middle is the B-51 end cap.... I milled a 3/8" diameter recess into it leaving 1/4" of the threads remaining, and then milled a 1/8" slot on the bottom side of the spring seat area.... On the right is the 8-32 adjusting screw, with a thinned nut and the adjuster with pin to prevent rotation.... You insert the screw through the threaded hole in the end cap (it just clears and can rotate) and then install the nut and tighten it against the shank of the screw, which leaves just enough space the screw can rotate but only have about 0.010" end float.... Then screw on the adjuster with the pin the slot in the end cap so it can't rotate....



Once assembled, there is 8 turns (1/4") of adjustment, and the only modification necessary on the stock was to file a small angled notch to clear the ball-end hex driver used to adjust it.... In the photo, it is roughly in the middle of its adjustment range.... When fully out, the adjuster takes up 0.10" of room in the spring seat, so I shortened the guide 1/4" to allow adjustment in both directions from the stock preload.... It is at the stock setting when 3 turns out from maximum preload.... I then tethered the gun to a tank through a regulator set to 200 bar (2900 psi) and shot 5 shots at each setting (1 turn increments) and plotted the average velocity as shown in the top graph below....



Since the stock preload was 3 turns out, and it was on the lower end of the knee of the velocity curve at 200 bar, I thought that would be a good place to shoot my first string.... The results are in the bottom graph above.... I was extremely pleased, as the first shot at 3000 psi ended up exactly 4% below the peak of 904 fps, and I got 32 shots averaging 888 fps (27.9 FPE) from 3000 psi down to 2220, which works out to 1.38 FPE/CI.... That is a great number at that power, one that few PCPs can match in my experience.... The question is why is this gun so efficient?....

There are a few things I noticed that are consistent with many of the things in "The Book".... First of all the initial, upwards part of the shot strings are rather ragged, with significant shot to shot variation (although within the 4% ES), compared to the downward, low pressure side of the string.... Secondly, the pressure drop is almost perfectly linear instead of curved upwards in the middle, which is normal for unregulated PCPs (they usually use more air per shot at low pressure).... Third, I noticed no "burping" and very little increase in the volume of the muzzle blast towards the end of the string.... Fourth, the sear has quite a strong spring so that it drags on the hammer to a significant degree.... Here is what I think is happening.... I think the heavily sprung sear is dragging so hard on the heavy, slow moving hammer, that that friction prevents it from bouncing and wasting air at the low pressure end of the string, and indeed you can't hear any resulting "burrpppp" from hammer bounce.... However, this drag, being not completely identical shot to shot, causes the velocity swings in the first part of the string, exactly where you would expect to see that effect exaggerated.... On the latter part of the string, even large changes in hammer strike will not change the velocity, and we don't see any.... The (almost complete?) lack of hammer bounce shows up in reduced air usage and greater efficiency towards the low pressure end of the string.... I think a lot of that is due to the (what we would think was excessive) sear drag on the hammer....

Now I have a problem.... Should I leave well enough alone and be happy with a fairly powerful and very efficient gun?.... or should I hog out the tiny ports and see if there is a beast sleeping inside this gentle giant?.... I think we all know the answer to that.... *grin*....

Bob

[rsterne]

SpiralGroove.... There is no simple answer to that question, it depends entirely on how where the valve is closing relative to barrel length.... In a MSP, with a dump valve, once you get to a reasonable power level the efficiency actually DROPS the higher the pressure because although you are increasing the velocity, you are dumping more and more air out the muzzle after Elvis has left the building, and the efficiency goes DOWN with increased pressure.... In an extremely efficient PCP, with the valve closing early (at 15-20% of barrel length or less) the efficiency may be almost directly proportional to the pressure.... double the pressure and double the efficiency.... Most PCPs will lie somewhere in between, about all you can say is that AT A GIVEN FPE LEVEL the efficiency is usually higher as you increase the pressure.... This is because to keep the same FPE you will reduce the valve dwell, using a shorter burst of higher pressure air, and using a greater percentage of the barrel to then expand that air....

Bob

[SpiralGroove]

Bob,
Very nice, can't wait until the next episode ------>>
Hail Caesar !!! 

[Motorhead]

Bob,
Just a piece of info that will help consistency .... VENT the backside of hammer by putting in a vent in rear cap 
Found coming into the side where cap rests within stock best. Small trough cut in the wood cut toward trigger mechanics gives path for air to come and go within this space as hammer strokes.

[rsterne]

I noticed mention in "The Book" of the vent holes in the piston just behind the striker lock screw, I guess they are supposed to allow air from the front of the piston to the back.... I was thinking of making sure the tube was vented front and rear anyway, thanks for the confirmation, I'll try that.... Now that I have removed the safety I may just drill through the hole in the end of the cap where the detent spring was.... I'll have a look....

Bob

[Motorhead]

I noticed mention in "The Book" of the vent holes in the piston just behind the striker lock screw, I guess they are supposed to allow air from the front of the piston to the back.... I was thinking of making sure the tube was vented front and rear anyway, thanks for the confirmation, I'll try that.... Now that I have removed the safety I may just drill through the hole in the end of the cap where the detent spring was.... I'll have a look....

Bob

Sprialgroove will confirm this i'm sure 

[SpiralGroove]

Hey Guys,
This venting "tweak" is pretty advanced stuff for me as I just got into 7th grade on PCP tuning
I saw something in the book about the valve not opening if the hammer was sealed (w/lube), but that went right over my now sharpened Pin Head.
So, I have no holes in my delrin hammer, besides the one drilled & tapped for my a 1/4" hex bolt turned striker.  No venting out the rear - what should I be noticing w/o?  Regulated shot string <2% ES.

Sorry Motorhead, no help from the undergrad

[Motorhead]

Lol ... have to go back into the PM's and re-read who it was then i was talking with about venting ... they did so with a LARGE gain in velocity and consistency.

My bad to insinuate such a thing !! 

Bob will get there ... he always does 

[rsterne]

In "The Book" they talk about screwing the striker in too far and blocking the vent holes behind it, resulting in a drop from 900 fps to 200.... because a vacuum is created behind the hammer.... When I had my gun apart I confirmed that yes, there is a pair of holes just behind the striker that pass into the center of the hammer, through the hole in the guide and spring, and would allow air into the space behind the hammer on firing.... If you made your own hammer and omitted those holes, I can see a problem for sure.... I had already thought ahead to that possibility.... However, adding more venting MAY decrease the shot-to-shot variation with the stock hammer, I don't know.... Has anyone found a difference in a stock, vented hammer with adding more venting to the rear of it?.... Obviously if you removed the safety (as I did) and didn't fit an RVA screw that area is well vented, and I was still getting a ragged front half of the string that way, if anything it is better with the RVA in place but that may have nothing to do with the amount of venting.... Anybody notice a difference in velocity or variation just by removing the safety?....

Bob

[calixt0]

Bob,  the things you write go right over my head about 85% of the time..Mostly because i dont' understand how airguns work. However I love the end result of what you do!   Keep up the good work..

[SpiralGroove]

Hey Bob/Scott,
So far I haven't gone with the rear velocity adjuster; trying to teach my Son the importance of a Safety (and to always use it before loading the gun).  Have pulled the hammer to make striker adjustments (which is a pain), but pretty much have the striker "in" almost all the way creating a hammer very similar to your shortened one.
So for the vent hole (even though I haven't experienced a power/shot consistency issue), I was thinking of drilling a 1/8" hole in the front and rear (not the tip holding the striker) of delrin hammer to vent any pressure which could create THE vacuum issue.

This should solve any problems -> Right?

Here are some pics........
http://i113.photobucket.com/albums/n227/kirk_schwarz/Hammer001_zps1d545311.jpg
http://i113.photobucket.com/albums/n227/kirk_schwarz/Hammer005_zps3a712c3a.jpg
http://i113.photobucket.com/albums/n227/kirk_schwarz/Hammer006_zps1315492a.jpg

[Pellgunfun]

Does the rifle loose a lot of weight when you replace the parts with Delrin parts, or not much?

[Motorhead]

Bob,
When i did all my work on the B51 a lighter hammer was indeed part of the rework. 
Pulling off the safety rocker opened the rear screw hole in a sense adding a SMALL vent.
What came to my attention was the air movement against my face upon firing ( have a LOT of facial hair )
In the case of the BAM what I was feeling was air intake at the 4mm hole.
Shooting over the Chrony finger over the hole / finger off the hole gave a variance flagging me to the fact suction was happening and the factory means to create venting was insufficient. 

Same thing happens within the Marauder rifle.  Solid diameter hammer absolutely need the rear caps adjuster hole to remain open for venting.

On nearly ever hammer i have made for my personal use has longitudinal cuts down there length with a ball end mill.  This allows transference of displaced air ahead of or behind the hammer.  Doing so have vastly reduced compression losses on the valve side and suction losses on the spring side.

 

[SpiralGroove]

Scott,
So for hammer venting, you're saying the 1/8" hole through the hammer (I was suggesting will be inadequate), and to go with some kind of groove down the hammer length instead ->  Right?

Kirk

[Motorhead]

Scott,
So for hammer venting, you're saying the 1/8" hole through the hammer (I was suggesting will be inadequate), and to go with some kind of groove down the hammer length instead ->  Right?

Kirk

On a BAM because hammer is rotationally floating ( IIRC ) having sear able to hold anywhere around entire diameter that groove trick DOES NOT work 

To get a sufficient vent took a 1/4" hole into side of end cap drilled on a diagonal angle intersecting the spring pocket area just behind shoulder.  The 4mm threaded hole was not enough. Also end cap safety screw location was IMO too susceptible to debris getting in. Which is why I did the vent hole where covered by stock and cut a trough in the wood to the trigger mechanics area. 

[SpiralGroove]

Hey Motorhead - Thanks !!

Hey Pellgunfun,
When I tuned my gun and reworked my stock, I probably lost 8 oz or 1/2 lb - a noticeable difference.
If I was to do the stock again, I would take more wood off the forearm which I left too bulky - also stain darker...
http://i113.photobucket.com/albums/n227/kirk_schwarz/BAMB50001_zps9996613a.jpg
http://i113.photobucket.com/albums/n227/kirk_schwarz/BAMB50-4_zps2f43e012.jpg

[Ribbonstone]

Have wondered about that vent hole.   I had my doubts at first, seeing the large port that the blot’s locking screw makes, and that’s ahead of the striker aseembly.  Thought that would vent any air being compressed ahead of the striker.


More going on that that.

Vent in the striker does seem to be needed (can plug it and find that out pretty quickly).  IS it because it compresses air ahead of it (but the locking bolt port on the left side should take care of that) or is it because it wants to pull a vacuum behind it?

So long as I’m wondering, if you use a lighter striker assembly traveling faster, would it need a larger vent than the heavy slow striker assembly?

Or is my brain too work-fried to see something simple?

[Motorhead]

Absolutely in terms of air transfer, faster a displacement happens faster any captive air wants to escape.
As flow resistance rises within the compressed or vacuum sides, greater is damping effect this air has on the motion of hammer.
Sadly were not cycling these parts in a vacuum where air pressure differential would not matter, so if we do what we can in mitigating these air motion damping losses more consistent will be the motion created by the mechanics of the system.   

[Ribbonstone]

ALMOST got me.

Was planning some tests with the vent open/vent closed/safety screw in place/safety screw removed.

Then I thought about it a little harder.

I like it the way it is and I’d better stop experimenting with it.

USUALLY, I’m the one to down tune a rifle for efficient air use, lower power, and a long shot count.  I happened to pick my BAM to be one of my more powerful PCP and wanted a pretty simple goal: +30 foot pounds and +20 sweet spot shots.

It’s not a rifle I plink with a lot, or pound long strings of targets.  Pretty much just a hunter that can keep energy up to about the 20 foot pound level at 50yards.

So unless Bob comes up with something that will drastically change my view point, am going to leave it alone.

[rsterne]

I think the air in front can vent just fine through that slot where the cocking pin rotates through, but SpiralGroove I can't believe you are getting away with Delrin against the trigger sear !!!.... Surely that can't last ?..... With no hole through the hammer and the safety installed, you will be getting a vacuum behind the hammer, particularly if it fits well.... 

I had removed the #26 screw to allow my hammer to rotate (as some said was OK in "The Book"), but have identified a problem with that.... If the hammer rotates to put that screw hole at the bottom (and it had) the hole can hit the sear on trigger release, and I think that was where my erratic velocities were coming from, at least to some extent.... I replaced that screw (so the hammer can't rotate) and when tested with the larger ports the erratic velocities had mostly gone away, and I am still just using the stock hole through the hammer to vent the area behind it....

Bob