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

[oldpro]

 That looks FANTASTIC and really the only upgrade would be the Acecare 500cc 4500psi carbon tank other than that its really about as good as it gets. Its a performer also!!! Not just a closet queen. Now its time to hit the field with it!!

[rsterne]

Yes, that would take about 1/2 lb. off the weight, and more than double the shot count.... All that would be needed to use it would be to swap out the 5/8"-18 NF tank adapter for one with 18 mm x 1.5 mm threads (which I have).... The gun is protected by a 3K burst disc, so no danger even if the regulator bypasses....

Bob

[MJP]

Nice clean lines, and good shot count too.

Marko

[rsterne]

Today I took apart the other lower, with the ART/SS valve in it, and cleaned it up and reinstalled the #87.5 jet that it came with.... After a couple of experimental settings of the hammer stroke and SSG preload and gap, I ended up using the 27 gram hammer with the striker set flush with the front face.... I lowered the preload to 0.37" (just 3 lbs.) and I was running low on my supply of 50 gr. JSBs, but I had a tin of the 45 gr. FX (JSB) pellets, so I used those to collect the data of velocity vs. preload, with the following results....



The combination of the stock jet and the very light hammer gave me a much nicer curve, with a full 200 fps of velocity adjustment before the valve fell off the cliff.... This is the widest range I have seen with the SS valve.... Once again, as soon as there was negative gap (preload) on the SSG, the gun became an instant air hog, and barked and kicked like a big angry dog.... However, as soon as there was any gap at all, the report was sharp and crisp, and just gradually became quieter as the velocity dropped, exactly like a conventional valve would do.... I set the SSG gap at 2 turns (just under 1/8") and tethered the gun to my 500 cc bottle with the output regulated at 1900 psi (actually just under).... I left the bottle tethered to the gauge of my Great White (tank valve closed), so that I could check the pressure drop to measure the efficiency.... I did two 7-shot mags. and got an average of 940 fps (88.3 FPE) with about a 1% ES, and the pressure dropped 250 psi for each mag., which works out to an efficiency of 1.18 FPE/CI.... Without changing any settings on the gun, I tried a 7-shot mag. with the 50.2 gr. JSBs I had remaining, and recorded an average velocity of 886 fps (87.5 FPE), but the pressure dropped only 200 psi.... Very surprised by that, I repeated the test, with exactly the same results.... That works out to 1.46 FPE/CI, which is pretty incredible for a .30 cal at that power level....

There is something different going on with this valve, that may be the key to some of the high efficiency numbers we are seeing.... I don't know if it has to do with the reduced velocity, or the slower pressure decay that causes in the valve throat.... but I have never seen such a drastic increase in efficiency before with only a 10% increase in pellet weight.... Normally you would expect the valve to release slightly less air with the heavier pellet, because it "uncovers" a shorter barrel length during the valve dwell, which essentially stays the same, and that results in a slight increase in efficiency.... For some reason, with the heavier pellet, this valve used less air.... a LOT less air, a drop of only 200 psi with the 50 gr. pellet compared to 250 psi with the 45 gr.... I am at a loss to explain why.... Perhaps the higher the pressure in the exhaust port the faster the valve closes, increasing the efficiency.... I don't know, but I like it.... 

I would like to increase the velocity with these two pellets a bit, so I think I will add a thin shim to the regulator to bump the setpoint a bit.... I would like to be able to adjust the velocity of the 50 gr. between 900-940 fps and the 45 gr. between 940-970 fps just by changing the SSG gap, and not have too big a gap (or too small).... I wish I had another Cobra tank block, with the externally adjustable regulator, but it is more important for that to be paired with the Cothran valve, which has no velocity adjustment expect by pressure.... At least I will have no worries about getting enough hammer strike with this valve.... the gun is very easy to cock, and I have lots of preload adjustment available, with only being at 3 lbs. at the moment....

My biggest problem now, is that I have been burning through pellets like crazy.... and so much air I had to rebuild my ShoeBox a couple of days ago (I got 90 hours on the HP O-rings in my Freedom 8, not bad).... Still, doing lots of shooting and collecting lots of valuable data is worth spending a few bucks, right?.... I hope my wife agrees.... 

Bob

[oldpro]

 I know I said this before but running a slightly smaller jet will close the valve faster for those lighter pellets and give you the same reaction as lowering the spring pre-load. More pellets/more air/ more time. Answers dont come cheap Im well aware of that. The Brod updates are my favorite thing on GTA right now no matter what valve etc. you use. Tons of useful data. The magic happens when the Jet and spring load are in harmony. One more thing Im probably going to tick off a few customers but with this light of a valve energy needed Im looking back at a internal SSG with light hammer for the Mrods. No cocking energy problem so now the SSG really shines again in a small space....aaaarrrgghhhh

[rsterne]

I agree, Travis.... I think an SSG based on the stock MRod spring, with only about 3 lbs. of preload (on the guide) would be great.... at least to start, for most tunes.... Using an SSG to avoid all chance of hammer bounce.... and combined with the easy cocking of this valve, and all the advantage of a light hammer.... just seems like a natural to me....

I have a gut feel (and that's all it is).... that by using an SSG with the right spring, preload and gap, and a light hammer.... the jet size may become less critical.... a bigger "sweet spot" if you like.... I don't need that, because there is no way I will ever tune more than 10% below the plateau velocity, because I don't want the velocity to rise below the setpoint.... I see NO reason to do that (other than for bragging rights on efficiency, which downslope tunes allow)....

I was asked tonight in a PM how far down the "knee" I tune, and it took me a while to answer, I had to look at a few examples to figure that out.... It turns out the my typical regulated tune is only about 3% below the plateau velocity at that pressure.... If you go down 5%, you start seeing a fair rise in velocity below the setpoint.... The lowest part of the knee I would tune to would be where the velocity peaks 1% above what it is regulated.... This allows extending the shot string 100-200 psi below the setpoint.... When tuning with an SSG, I like to be able to JUST reach the plateau when the gap hits zero.... In the case of the graph above, I'm not quite there, because when I dial in preload I get a 10 fps jump up from 960 to 970.... However, the ART/SS valve has a very "soft knee" to the velocity curve, it takes a lot of increase in the gap to drop from the plateau to the downslope (in this case the cliff).... so I don't need to be able to get to the plateau at zero gap.... With a conventional valve, there is often only a couple of turns of the SSG between the plateau and the downslope.... With most conventional valves, the downslope starts about 7-10% below the plateau velocity.... That means that for any given pressure, the "best" range to tune within is only 10% wide at most.... If you want less velocity, you are better off to reduce the pressure.... and if you need more power, then you have to increase it....

I was concentrating on trying to get a wider velocity adjustment range with the ART/SS valve.... but in fact for use in a regulated gun, a tunable range of 100 fps is all you really need.... I wouldn't be surprised if the same thing applies unregulated.... because just like with a conventional valve in an unregulated PCP.... if you tune the velocity down very much (with a lighter hammer strike), you have to reduce the fill pressure or the ES gets too great.... I looked at a fair amount of my data tonight, and a bell-curve for a 4% ES typically starts about 8-10% below the peak velocity at that pressure (near the top of the downslope).... If you want a narrower ES, you have to start even closer to the maximum.... I have a feeling that the ART/SS valve will be the same....

The Cothran valve has basically no velocity tuning range, it is governed solely by pressure.... A conventional valve can be tuned over a huge velocity range (from flat out to valve lock).... BUT only a small portion of that range is USABLE unless you reduce the fill pressure (or are prepared to accept a huge ES).... Once you realize that everything happens within about a 10% range of velocity.... and to tune outside that range you must change the pressure.... it makes a LOT more sense....

Isn't learning something new wonderful.... I feel like tonight was one of those rare "lightbulb moments".... 

Bob

[rsterne]

I had a bit of an epiphany last night about how to tune this valve.... When I looked back over how far below the plateau velocity I generally tune my regulated guns, it turns out that about 3-4% is typical.... By the time you are 5% or more below the plateau, the velocity starts to show a significant increase when the pressure drops below the setpoint.... If you tune less than 3% below the plateau, the gun tends to use more air for not much increase in velocity.... and of course at the plateau it becomes an air hog.... So, this morning I hooked by regulated 500 cc bottle to the .30 cal BRod with the ART/SS valve in it, and did some experimenting.... but I made one (hopefully last) change first.... I made a slightly heavier hammer, that weighs 39 grams instead of the 27 gram I was running yesterday.... This is in between that and the 51 gram hammer I installed in the BRod with the Cothran valve.... The reason for the slightly heavier hammer is that I could not quite get to the plateau velocity with the 27 gram hammer without decreasing the SSG gap below zero (ie adding preload).... which wasted HUGE amounts of air.... I wanted to be able to get to the plateau at zero gap, and adding a bit of weight to the hammer seemed the best way.... and it worked great....

I had checked the output of my 500 cc regulated bottle, and it was actually 1850 psi instead of 1900.... Using that, I plotted a new velocity vs. preload curve with both the 45 gr. and 50 gr. pellets, and got a plateau of 972 fps with the 45 gr. and 945 fps with the 50 gr.... The valve fell off the cliff at 9 turns of gap, with an adjustable range of about 150 fps before it did that.... I tried a few gap settings around 3-6 turns out, and found that at 5 turns of gap, if I let the tank pressure fall below the 1850 psi setpoint I was getting a slight bump in velocity, just as I expected, peaking at about 1780 psi before declining.... That is EXACTLY what I was looking for, my normal tuning point where the shot count is extended a bit below the setpoint pressure, but with a velocity increase of less than 1%.... The problem was, it was occurring at about 940 fps with the 45 gr. and about 880 fps with the 50 gr. pellets, and I wanted a bit more than that.... The solution was to drain the bottle and increase the setpoint pressure a bit.... On the third try I got the shim stack just right, and the new setpoint is exactly 1950 psi.... I repeated the testing with this higher setpoint, with the following results....



As you can see, the plateau velocity is higher, as you would expect with the 100 psi higher setpoint.... Is it now 988 fps with the 45.0 gr. pellets (97.6 FPE) and 954 fps with the 50.2 gr. (101.5 FPE).... and I can get there with a turn of gap in the SSG.... I found that with 4 turns of gap the velocity didn't increase below the setpoint, but it didn't fall off immediately either, giving me another 5 solid, usable shots below the setpoint.... the velocity didn't drop off until the pressure fell below 1800 psi.... What I had done was tune the gun to act like it was unregulated below the setpoint, with the velocity peaking about 100 psi below the setpoint.... but the curve is so flat at that point, the slight bump in velocity is lost in the SD.... The fact is, I have the gun tuned at 1950 psi, but I can use the tank down to 1800 and never even notice it.... all the shots stay within the normal ES, which is just over 1% (about 12 fps with unsorted pellets)..... The velocity with the 45 gr. pellets is just over 3% below the plateau, and with the 50 gr. pellets it is right at 4% down.... Once again, my preferred tune on a regulated PCP is right around 3-4% below the plateau.... I did one 7-shot mag. as an efficiency test with the 50.2 gr. pellets, and it worked out to 1.25 FPE/CI at an average of 916 fps (93.6 FPE).... This is not as good as yesterday, but the velocity is 30 fps higher, so pretty much expected.... I should still get about 35 shots at nearly 94 FPE with this tune.... and by using a 4500 psi bottle that would more than double....

Bob

[rsterne]

Here is what the .30 cal. looks like assembled....



It weighs 3 oz. more than the AR version.... The stock is heavier, but the barrel is lighter because of the CF sleeve to bring the diameter of the TJ's liner up to the same as the LW barrel (16mm).... all up with scope and rings it is 9 lbs. 11 oz.... I'm quite pleased with the appearance and ergonomics.... and it balances well, right where you would hold it near the front of the wooden stock....

Bob

[KnifeMaker]

A Wonderful Job Mr. Bob! and a huge At-A-Boy!!!  (Texan for congrats)

Knife!

[YEMX]

Bob, that is beautiful!!!  What kind of stock is that?

[rsterne]

No idea.... I got it in a box of goodies from Travis last winter when he was cleaning/moving shop.... I think it was intended for a Bullpup, but they are illegal in Canada.... so it became a rifle stock by inletting it for an MRod trigger in the normal configuration.... It is quite light....

Bob

[oldpro]

 P12 bullpup stock.

[rsterne]

No, you must be mistaken, Travis.... it's a RIFLE stock (now).... 

Bob

[oldpro]

My bad your right Rifle stock.

[Buldawg76]

I think it look quite nice how its setup right now. I guess its a bullrifle now.

Mike

[oldpro]

I think it look quite nice how its setup right now. I guess its a bullrifle now.

Mike

Bobs Bullrifle That has a nice ring to it.

[Buldawg76]

There ya go

[YEMX]

P12 bullpup stock.

That's what I thought- Very nice re-hash of it Bob!!  I'm liking the semi-bullpup RIFLE set up more and more...

[rsterne]

I spent the last 3 days casting bullets from all the moulds I got from NOE over the summer.... I went through about 20 lbs. of 40:1 lead:tin alloy from Rotometals, and cast bullets in .224, .243, .250 and .257 cal, and one in .308.... One of the bullets I cast was the heaviest BBT designed for .25 cal airgun barrels.... It turned out to be 58.3 gr. in that alloy for the FN version, and 55.0 gr. for the HP.... Here is what one looks like after pushing through the LW Polygonal barrel on my BRod....



The back of the nose measured 0.250" and the driving band 0.252" before pushing through.... After pushing through the choke, the driving band measured 0.251" across the grooves, and 0.246" across the "lands", but they did not drag on the center portion of the bullet, exactly the way it was designed to work.... At 1950 psi the HP just broke 800 fps, but I wanted to find out what it would do with the SS Valve at 3000 psi, so I swapped parts around to test that.... With the SSG adjusted for a turn of negative gap (preload) I got 952 fps with the FN (117 FPE) with a huge BRRAAAPPPP of air, but I assume that is the plateau velocity.... With a whisker of gap (less than 0.020") the report was normal, nice and crisp (although pretty loud) and I recorded 930 fps for the FN (112 FPE) and 950 fps for the HP (110 FPE).... which is pretty good for the stock length LW barrel of just 23.8".... This gives a pretty good idea of what the SS valve is capable of.... The only way to get more would be to use a retracting bolt to provide unobstructed flow in the chamber, as right now it is the equivalent of 0.234", and I am actually using that for a transfer port diameter, as there is no sense in going larger....

This experiment gives significant insight into what could be done with the ART/SS Valve in .257 cal with a longer barrel, using a 4500 psi bottle regulated down to 3000 psi.... With a 28" barrel, 130-140 FPE should be quite possible, which would enable using the venerable Lyman 257420, which is the benchmark bullet in .257s.... My Hayabusa .257 (3000 psi fill, unregulated) with those specifications (with retracting bolt) reached a peak of 160 FPE, and can be tuned for a very nice string at over 130 FPE.... and that could be increased a bit if regulated to a solid 140 FPE.... There is no reason that the SS valve could not produce the same results, and be a LOT easier to cock as well.... My Hayabusa uses a 115 gram hammer, with a stroke of 1.2", and an 18 lb/in spring.... Cocking force at 160 FPE is 24 lbs.... 

The experimenting I have done here provides a roadmap for anyone wishing to build a wonderful Varmint or long range target rifle, using proven components.... A 1/2" OD TJ's 25-20 barrel that is 28" long, with a 14" twist, the MRod action (modded for a retracting bolt), with an ART/SS Valve, and a 500 cc CF bottle regulated down to 3000 psi would supply the power.... Tensioning or sleeving the barrel to stiffen it, and maybe a harmonic tuner, should provide the accuracy.... Some decent optics, and an SCBA tank (for refills or tethering), and you would be good for a whole day of serious long-range Varmint busting....

Bob

[rsterne]

Ran some test today with the .25 cal BRod tethered at 3000 psi.... still running the 12 lb/in spring with only 3 lbs. preload.... I was able to just reach the plateau at zero SSG gap as this pressure.... using my 47.5 gr. BBT HPs I got 1002 fps, and with the 50.4 gr. BBT FN I hit 977 fps (106-107 FPE).... At 3.5 turns of gap that dropped to 100 FPE with a nice crisp report.... I didn't check the efficiency.... but those HPs at 970 fps would sure lay a smackdown on anything up to Coyote size, I think....

Interestingly, this is very close to the same SSG setting I was using for the .30 cal BRod shooting 45-50 gr. pellets on 1950 psi.... and only a couple of turns less gap than the .25 cal shooting 34.1 gr. Kings on 1900 psi.... Once you find the sweet spot on the ART/SS valve it doesn't seem to require a lot of adjustment.... even with large changes of caliber or pressure....

Bob