Tale of THREE BRods - .25, .30 and .357 Completed, Page 19 !

[MJP]

Thats a nice find Bob, gets me thinking about couple of my guns too.
Nice builds by the way, the one with ar stock and crosman group looks lot like my first gun did, and I still like it a lot.

Marko

[rsterne]

Oh, only one of them will be a .25 cal.... the other will be either .30 or .357.... although if I had an extra .257 barrel with a 14" twist about 28" long, I would give serious consideration to using that with a regulator set around 3000 psi on my 250 bar bottle with the Cothran valve.... my own version of the WAR Hammer, just to light a fire under Travis.... 

Bob

[oldpro]

Haha you old goat you'll have to do better than that to ruffle my feathers LOL. Seriously though let's do the 257 with Dons valve so you can finally convince me to build one. I'm on the fence just need a push.

[oldpro]

Just a little push LOL

[grand-galop]

Just a quick note on the PHYSICS...  I dig my 30+ years book and it also says that once an object is in mouvement, there is a cosine math to apply for the resistance of gravity.. Since the object is moving 90 degree OFF gravity pressure, it would translate to only 30 pourcent of the initial pressure required to move the same object..    Just me saying. And if you were to shoot down with the same line of forces as the gravity, you would substract the aid of the gravity to move the object for the force required to move it..

[rsterne]

~taking a hefty swing at the fencepost with an axe~

TIMBER !!!!

Bob

[grand-galop]

LOL

[rsterne]

I pulled my modded valve out of the tube today and bored out the exhaust port to 0.234", and drilled out the transfer port to that diameter as well.... I inspected the MRod poppet, and it has a nice shiny seat, just indented a few thou.... It is not showing any sign of extruding into the port, as I have seen happen with a 0.281" throat.... The throat in my current valve is 0.266", which I have also used before without issue at 3000 psi.... However, I don't want to go any larger.... Unless you change the poppet material, going larger on the port, or using more than 3000 psi, is just asking for trouble.... The throat (when you subtract the 1/8" stem), the exhaust port, the transfer port, the barrel port and the bore (when you subtract the 3/32" bolt probe) are now all the same area.... I consider this the maximum practical for a modded MRod valve, unless you change the poppet, and no point in doing that unless you also go to a probeless bolt....

The larger ports increased the maximum velocity at 1900 psi by 24 fps, from 956 to 980 fps.... Here is the comparison, before and after enlarging the ports....



I want the ability to achieve 950 fps on the knee of the curve, and with the 0.219" ports I would have needed to increase the regulator setpoint to 2000 psi.... Now I can use 1900 psi and get there.... I adjusted the gap to 3.5 turns out, and shot the following 10-shot string using the 34.2 gr JSB King Heavies, tethered at 1900 psi....

960
959
956
959
959
957
957
960
955
957

ES = 5 fps = 0.5 %
958 fps average
69.7 FPE average

This achieves my goal with these pellets.... I don't have an efficiency yet, that will have to wait until I install the bottle.... I have not yet decided if I will use my valve, or the Cothran, for the regulated .25 cal, either will work perfectly.... The edge for the Cothran valve over mine is now just 10-20 fps at 1900 psi.... depending on how I adjust the SSG gap.... To adjust the velocity on the Cothran valve, you have to change the pressure, as for best efficiency you need to operate it just above the cliff.... It would make sense to use whichever proved to be the most efficient at the velocity I end up shooting.... That has yet to be determined....

Bob

[oldpro]

Splendid as usual! Would love a video of those dogs barking.

[grand-galop]

is that mean that to get to close to zero gap, you could use a weaker spring??  Those number are COLOSSAL...

[rsterne]

Absolutely could use a weaker spring on the SSG.... At 3.5 turns out on the gap (actually about 0.20"), the cocking force is a comfortable 14 lbs.... I am currently using a 0.059" wire 0.48" OD Century spring that is 3" long that is 13 lb/in, and they make a 0.055" at 11 lb/in and a 0.051" wire at 8 lb/in.... either of which will work at 1900 psi.... With the lightest one, the cocking effort should be about 11 lbs. and the gap about 1 turn.... I could also use a much lighter hammer in the regulated one, of course.... and I may try that next, just for giggles.... I'm curious how much an MDS hammer might decrease the gap?....

Bob

[grand-galop]

Interesting!!! Since you use your current setup and going with the bottle, i taught you were to regulated it anyway.. The advantage of the back screw is you can at will, change the spring in second.. The bugs are fading away and numbers are great efficiency Wise.. Thanks

[rsterne]

I will likely build one with a regulator and one without.... The .25 cal is looking like it will be the regulated version.... although when I try it with bullets in the 40-65 gr. range at higher pressures, that could change.... The other gun will be either .30 or .357 cal.... The nice thing is that will all the parts interchangeable, I have tons of options.... and of course I could always get another reg. should I decide to regulate both guns....

Bob

[rsterne]

I've been hearing all about these MDS Nylon lightweight hammers, and this gun, with the current hammer spring, needed a large gap on the SSG, so it seemed a good candidate to try one.... I made one of identical dimensions to the steel one I had, but used a 3/4" diam. steel core and the rest is MDS.... I simply knurled a piece of 3/4" steel, drilled it for the hammer spring, and then bored out a piece of MDS of the correct length to be a press fit, and pressed them together.... I then machined the MDS to the correct diameter, cut off the excess steel at the front and turned it to length, and drilled and tapped the 8-32 hole for the cocking pin through both, then machined the angle for the sear in the MDS.... The finished weight was 51 grams, almost exactly 50% of the original steel one, which was 104 gr.... I installed it in the tube with my valve in it, and tethered the gun at 1900 psi, to duplicate the test I did yesterday of the 0.234" ports.... I used the same 34.2 gr. JSB Heavy pellets, so the ONLY change was the hammer.... Here are the results....



With zero gap, the velocity was already starting to fall below the maximum, at 963 fps.... This is virtually identical to the velocity I got with the heavy hammer at 3 turns of gap.... The velocity at 1 turn out with the light hammer was almost the same as at 4 turns out with the heavy hammer as well.... Therefore, reducing the hammer weight by 50% required a 3 turn decrease in gap to obtain the same velocity.... The hammer spring I am using has a rate of 13 lb/in, and 3 turns of the gap adjuster, which is 18 TPI, is 0.165".... That means that the light hammer requires just over 2 lbs. more force to cock the gun, and 0.165" more hammer travel, to obtain the same results....

Using Lloyd's hammer spreadsheet, I calculated the hammer velocity at the instant it left the SSG, and from that the hammer energy and momentum.... To do that, I had to include the weight of the SSG guide (22 grams) during acceleration, to get the velocity, and then subtract it to get the energy and momentum of the hammers.... Here are the results that produced identical (963 fps) velocity and FPE in the pellet, which is what matters, of course....

104 gr. Hammer
Velocity 11.54 fps
Energy 0.471 ft.lb.
Momentum 0.082 ft.lb/sec
Lock time 10.1 mSec
Cocking Force 14.4 lbs.

51 gr. Hammer
Velocity 17.83 fps
Energy 0.554 ft.lb.
Momentum 0.062 ft.lb/sec
Lock Time 7.1 mSec
Cocking Force 16.6 lbs.

These results are skewed somewhat by the admittedly heavy SSG assembly in this gun.... as it represents a much higher percentage of the weight of the light hammer than the heavy one.... However, in this particular application, that's what I got.... There is no question that this lightweight hammer will work in this gun, with either valve, at 1900 psi with the 34 gr. JSBs, it would just be a matter of tuning it for the velocity you want.... There is also no question that it requires about a 15% increase in cocking force to get the same performance.... I expected an increase, but it's nice to actually put some numbers to it.... Bear in mind, this is a pretty powerful .25 cal, delivering 70 FPE at only 1900 psi....

Although I did not try it at 2900 psi, the results I could expect were pretty obvious.... I was unable to max. the velocity out with the 34 gr. JSBs at that pressure with the heavy hammer and 0.40" of preload on the spring.... I think I could just get there with 0.60" of preload at zero gap, which would put the cocking force at about 19 lbs.... With the light hammer, I'm pretty sure I couldn't come close to maxing the velocity using an SSG, I would have to go to a preloaded spring.... Using heavier bullets at 2900 psi it would be even harder to flow enough air to max. them out, even with the heavy hammer.... Even the Cothran valve had to have the SSG dialed down to zero gap with the heavy hammer, to get to maximum velocity with my 51 gr. BBTs....

My conclusion is that the MDS hammers certainly have their place, and can do wonderful things at moderate power and pressures.... However, when you are trying to extract maximum FPE, particularly with heavy bullets and at high pressure.... a heavy hammer helps you get the hammer strike required without using a spring that make the gun too hard to cock.... The Cothran Powerhouse valve, with its ability to open with a much lighter hammer strike, can certainly make use of lighter hammers, however.... I have done no efficiency testing with either hammer or valve, so can draw no conclusions regarding that at this time....

Bob

[ax57]

I realize that you haven't done efficiency testing yet, but would this lightweight hammer be the route to go if one was happy with the current velocity and FPE, and was now looking to maximize their shot count?

[shorty]

Well done with your build Bob. I am enjoying every step your taking with it. I couldn't be any more happier seeing you ring this platform out.

I know you have always been the disco fan or "custom build" but being your first Mrod build with the vast history of what has been done is a great feeling seeing what your finally doing with it and providing super data to support it.

I actually can't wait to see what happens when you throw that 30 cal barrel on it.

Too be honest, I haven't seen any of your magic on it yet as a very few of us have done what you already have done BUT, I know you gonna push and twist the screw driver in soon and we are gonna be in amazement.....

[rsterne]

Stay tuned.... I frankly have NO idea.... and it very likely depends COMPLETELY on the type of tune you have.... Scott (Motorhead) has spectacular success with low-medium FPE tunes using a light hammer in regulated PCPs, so who am I to argue with success.... The results he is getting may or may not apply to my BRods, with either valve.... I hope to find out if they apply to a regulated PCP shooting at 70 FPE on 1900 psi....

Tim, there is only so much power to be had with a given barrel length and pressure, you know that.... If I was able to break new ground there, we would have to be looking at some new theories.... Still, not too many have reached over 70 FPE with a 34 gr. pellet on just 1900 psi in .25 cal.... I seem to recall a chart you posted just a short time ago that claimed 48 FPE at 1900 psi was about the maximum?.... http://www.gatewaytoairguns.org/GTA/index.php?topic=116706.20 .... I realize that was with 0.187" ports, and I'm at 0.234".... but seems to me that is an improvement, no?.... Using your numbers, with a 0.234" port, you would have predicted about 60 FPE at 1900 psi, if I extrapolated your spreadsheet correctly?....

Bob

[shorty]

Yup, pretty much true and proved the 60 fpe tune at 1900psi using 43 eujins with the stock barrel using .196" porting.

I definitely see an improvement with your system. That's what I am point out... I think you nailed the plenum size down and the power plant.

Great work and can't wait to see more.

[Motorhead]

Bob ...
Can't but wonder in an M-rod that the MDS hammer cocks a LOT easier & smoother too !
By chance did you side flute the MDS hammer to vent it ?
Having an SSG plugging the rear of Vel adjuster that in stock form doubles as a suction vent for hammer bore.

[Machinist]

The calculations you and Lloyd make boggle the mind.

Lock time 10.1 mSec

Steve