Brod build: 100 FPE; stock barrel; shot pic updated

[Kinetic45^]

This is the one I got, in black....  https://www.aliexpress.com/item/500cc-Carbon-Fiber-HPA-Tank-Black-4500psi-Working-Pressure/32817495698.html?spm=a2g0s.9042311.0.0.vM1q66
Bob

HaHa, same one I just got for my WARP w/ 17" FLEX barrel!

Still have not decided about a barrel band, vs tensioning a barrel, vs floating it (with potential harmonic weights). For sure I will start with it just floating, and see how it shoots. Then I will decide.

My rifles get knocked around in real world hunting and I had problems with changing POI with the stock band so I went with the Hill which has a snug fit inside an o-ring.  And it has a Picatinny rail at the bottom for flashlight or laser.  Not sure a band would help that much being brought back towards the breach to clear the bottle would help but I probably would anyway

at first, on my Synrod .25 I used a whole slew of o-rings mounted on the barrel inside the shroud (like 20 at random spacing) to dampen and it does 'sort of work' as a 25 group, 5 shot each, before and 25 after group test had slightly smaller aggregate numbers, but small enough that it could just be statistical random noise too, HaHa.

I am a fan of tensioned barrels and did my first one on a BAM B-51 back in 6/04, shrouding with integral LDC too.  It really sold me because the change in accuracy was phenomenal.
A collar at the breach and threaded air stripper on the muzzle with an aluminum tension column is how I would go.  Everyone uses CF nowdays but I see aluminum as almost as light and SO much cheaper.  I thickly grease up the steel barrel and slide the aluminum jacket on and tension then install in the gun.  The regular shroud fits over the tensioned barrel and still has a small airspace for the air stripper to dump to.

[PCPhack]

Carl, what you described for the tensioning is pretty much what I envisioned. Good to hear that it will work out well. I also currently use a Hill band on the .25, but as you know, it can't really be used after the bottle modification. I thought about a custom machined band that could clamp around the bottle and the barrel shroud as well. It would need to have close tolerances. Obviously, it can't dig into the bottle whosoever.

[rsterne]

I would look at a way to use the outer shroud to tension the barrel and still absorb the air to collect that noisy lead dust.... An arrangement like this possibly?....









I call that my "Vented Barrel Tensioner" or VBT.... I think it's pretty obvious how it works, no?.... You can't have baffle in front of it.... but it allows the shroud to work to absorb some of the residual muzzle pressure at least.... and only extends the barrel about 3" or so.... The tension is easily adjustable to change the barrel harmonics, and the Belleville washers provide much better temperature stability, always a concern with barrel tensioning systems....

Bob

[PCPhack]

Very cool Bob. I love to see custom machined parts. I had not even thought of the temperature fluctuation impacts to tensioning. I will definitely come back and update the thread here as I progress.

[rsterne]

Unless you use identical materials for the barrel and shroud, temperature change makes a HUGE difference in tension.... If the barrel expands more than the shroud, it can cause complete loss of tension without the Bellevilles.... The longer the Belleville stack, the less percentage change in tension with temperature.... If the shroud expands more than the barrel, the tension increases, of course.... but again the Bellevilles minimize that (providing they don't go flat and bottom out)....

Bob

[Kinetic45^]

Unless you use identical materials for the barrel and shroud, temperature change makes a HUGE difference in tension.... If the barrel expands more than the shroud, it can cause complete loss of tension without the Bellevilles.... The longer the Belleville stack, the less percentage change in tension with temperature.... If the shroud expands more than the barrel, the tension increases, of course.... but again the Bellevilles minimize that (providing they don't go flat and bottom out)....

Bob

So how many Bellevilles do you usually go for with a steel barrel and aluminum shroud?
I used two in my first one (the B-51 and have used three in other rifles but you are showing a lot more in your picture, and that is with a carbon fiber shroud which has a low coefficient of expansion, so basically just the steel to compensate for.

[PCPhack]

Given the concerns over temperature changes, I wonder if one can obtain just as good accuracy without the tensioning concerns by using barrel weights at appropriate harmonic node points. Or is the tensioning approach so much better, that even with temperature fluctuations, it is better than such methods? Can you Bob, or someone more knowledgeable on this comment?

[YEMX]

I would think a barrel tensioning system would be easier- just crank on it little by little in either direction to increase or decrease tension- the barrel weight I think would be a touch more work, as you would have to slide it incrementally over the barrel whilst shooting groups...  Unless there's a way to calculate how a barrel flexes during the shot cycle...  JMHO.  Either way you're tweaking and shooting groups until you find your settings- so I guess it's a toss up, really.

[rsterne]

Harmonic tuners can work great, but are 100% trial and error (as is finding the optimum tension).... There are two basic types, the slide-over "limb-saver" type rubber bushing, which you tune by sliding along while watching group size.... or the muzzle mounted, adjustable position weight, like rimfire shooter use.... It consists of a threaded weight (usually with locking collar or screw) that you move a tiny amount to fine tune the group size.... I understand that just a few thou can make a huge difference.... I plan on trying them on my current 6mm and .257 builds, which will be using 14mm barrels with huge 20mm OD CF sleeves bonded to them.... so they will already be incredibly still.... The harmonic tuner will, hopefully, be the icing on the cake....



Using 28 TPI has a reason.... each 30 deg. of rotation (1 hr. on a clock face) is 0.003", with 10 deg. being 0.001".... I don't know how you could use a limb-saver on a barrel inside a shroud.... but a muzzle (shroud) mounted tuning weight might work, I don't know.... One thing to be aware of.... changing the velocity or bullet can require retuning, regardless of which type of tuner you use (even tensioning)....

Bob

[PCPhack]

The tank is arriving next week from China.

I ordered the WAR regulator at the same time, and the shipping date keeps getting pushed back. I am not sure about the reasons for delay. There was available stock the day I ordered.   

[PCPhack]

Jim was able to finally ship my regulator today from WAR. Great.

Now I still need to return one piece of the chassis that arrived damaged for replacement.

Small steps...

[PCPhack]

I have the regulator and carbon fiber bottle in hand.

The chassis did arrive, but is being swapped, due to it arriving with a scratch. The reseller has been great about the process so far. The RAI chassis is sharp at the corners up by the front, and one of them had penetrated the shipping box.

I did get a chance to measure where the chassis ends on the air tube. If I have JSAR make me a custom length tube that ends right where the chassis ends, so that the regulator block is the very next thing, without a gap, then the calculated plenum volume, with SS valve, and counting the smaller cylinder shape cut into the regulator theaded base, is 119CC.

119CC should allow me to reach my final goals with the gun.

I would like to reach 120 FPE if staying at .25 caliber, and a bit more if I jump to a larger and longer barrel. As I don't mind to run the regulator up around 3000psi, with 4500 in the carbon tank, I think this is realistic. The real aim here is a .22 rimfire replacement power wise.

In the gun which serves as the basis for this (.25 SS valve, .1875 porting, mds hammer, etc) I am up to around 85-90 FPE maximum. I think I will need to move beyond the .1875 ports, and do some probe (and possibly breech) work, to get the last bit of power out.

Comments welcome. I have not done any more calculations (yes, I should), only stating what the gun currently does, and where I want to go.

[rsterne]

I managed 117 FPE today (952 fps with 58.3 gr. BBT-FN) with my .25 cal BRod, running on 3000 psi with the 23.8" LW barrel.... The SS valve has a 1/4" port on a 15 deg. angle, the bolt probe is 3/32", which makes the area of the chamber the same as a 0.234" hole, so that is what I used for the transfer port.... This was for a dump shot, backed off a bit it shot 930 with the FN and 950 fps with the 55 gr. HP (ie 110-112 FPE).... I think this is the maximum at that barrel length without using a retracting bolt to increase the chamber area to full bore-size and using a matching 1/4" transfer port (6.8% more area for flow).... That alone would be good for a few FPE.... and of course a .257 cal barrel is 5.6% more bore area as well.... Without going to more pressure, a 28" barrel (17.6% longer) is the next logical step to find more FPE.... Multiply all those gains together and you have a third more FPE, which sounds like a lot, but my .257 Hayabusa, which met all those specs. maxed out at 160 FPE at 3000 psi, and gets a nice string unregulated at over 130 FPE.... Tethered or regulated at 3000 psi, a nice solid 140 FPE is possible without being too hard on air....

Bob

[PCPhack]

I managed 117 FPE today (952 fps with 58.3 gr. BBT-FN) with my .25 cal BRod, running on 3000 psi with the 23.8" LW barrel.... The SS valve has a 1/4" port on a 15 deg. angle, the bolt probe is 3/32", which makes the area of the chamber the same as a 0.234" hole, so that is what I used for the transfer port.... This was for a dump shot, backed off a bit it shot 930 with the FN and 950 fps with the 55 gr. HP (ie 110-112 FPE).... I think this is the maximum at that barrel length without using a retracting bolt to increase the chamber area to full bore-size and using a matching 1/4" transfer port (6.8% more area for flow).... That alone would be good for a few FPE.... and of course a .257 cal barrel is 5.6% more bore area as well.... Without going to more pressure, a 28" barrel (17.6% longer) is the next logical step to find more FPE.... Multiply all those gains together and you have a third more FPE, which sounds like a lot, but my .257 Hayabusa, which met all those specs. maxed out at 160 FPE at 3000 psi, and gets a nice string unregulated at over 130 FPE.... Tethered or regulated at 3000 psi, a nice solid 140 FPE is possible without being too hard on air....

Bob

Thanks Bob, I saw your other thread and am now pondering plenum. I might should go more than the 119cc I would get with the nicest aesthetics of the block against the chassis, but chasing after power, I get the feeling I need to get it up around 150, so there would be a 5 to 6 centimeter gap between the chassis and start of the regulator block.

[rsterne]

You will gain a small amount by having more than 1 cc per FPE.... but I never bother.... The more efficient the gun, the less you gain from a larger plenum.... The plenum in my BRod is 85 cc.... in my Hayabusa it is the entire 460 cc reservoir....

You mentioned early in the thread using a stock MRod .25 cal barrel, which I believe are only 20" long.... That will really limit your FPE if you do that....

Bob

[oldpro]

 I agree the 20 inch barrel regardless of porting will be very hard to push past 100 FPE without really ramping up the pressure to dangerous levels. To get you where you want you will need full bore porting (.250)and 24-28 barrel and some heavy lead.

[rsterne]

If you are producing 120 FPE at 1.00 FPE/CI, you use 120 standard CI of air to make the shot.... at 3000 psi, that is only 9.5 cc.... With a 120 cc plenum it would drop to 2762 psi during the shot, and the average would be 2881 psi.... With a 150 cc plenum the pressure would drop from 3000 psi to 2810 psi during the shot, with the average being 2905 psi.... With the entire 460 cc available in my Hayabusa, it only drops to 2938 psi, for an average of 2969 psi.... It is the difference in the average pressure that you use to calculate the potential FPE.... The 120 cc plenum loses 3% compared to the unregulated Hayabusa.... Going to a 150 cc plenum from 120 cc would only buy you back about 0.8%, which is only 1 FPE on a 120 FPE gun....

Bob

[PCPhack]

If you are producing 120 FPE at 1.00 FPE/CI, you use 120 standard CI of air to make the shot.... at 3000 psi, that is only 9.5 cc.... With a 120 cc plenum it would drop to 2762 psi during the shot, and the average would be 2881 psi.... With a 150 cc plenum the pressure would drop from 3000 psi to 2810 psi during the shot, with the average being 2905 psi.... With the entire 460 cc available in my Hayabusa, it only drops to 2938 psi, for an average of 2969 psi.... It is the difference in the average pressure that you use to calculate the potential FPE.... The 120 cc plenum loses 3% compared to the unregulated Hayabusa.... Going to a 150 cc plenum from 120 cc would only buy you back about 0.8%, which is only 1 FPE on a 120 FPE gun....

Bob

Well thanks for that, and running the math.

[PCPhack]

I received the new regulator and set it up with my .25 Marauder for basic testing. It held 3000 PSI for a few days, but today a few hours after filling the bottle up to 4000 PSI, it sprung a fast leak from the adjustment screw hole. So it will have to go back to WAR, or if Jim gives the go ahead (as to not void any warranty), I will ppen it up.

My scratched RAI chassis is back at the vendor, so the replacement ine for that should come in another week.

[KnifeMaker]

I have the same bottle that bob linked. It has a nice gel coat that looks very nice indeed. I've been shooting it for two years now. I fill to 3600 for the .257 Talondor.

I tried a .25 barrel, well several factory walther bbls. Both choked, and with the choke removed. I ended up going with a TJ .257 BBL, and am very glad I did. Ultra accurate, and the selection of bullets is very good.

I cast bullets for it that are anywhere from 56 gr all the way to 110 grain. However, I recently shortened the 110 spire point down to 84 grains, and it is a real laser!

It probably wouldn't hurt to put a carbon fiber sleeve on it for rigidity, as it is 24" long, and is shooting in the 180 fpe range with Dyotats valve, and139 fpe with R&L's valve. 

The .257 has proven itself to be one of the best short to long range cal's out there by quite a few shooters. Not once have I seen bad accuracy from a .257 matched to a TJ barrel.

Knife