Building a "Friendly" .30 cal M-Rod with Cothran and Huma

[rsterne]

Nice improvement, Lloyd.... I agree with Travis, when you open up the transfer port and up the FPE, you will wish you had a bigger plenum.... There are a ton of ways to tune, it just depends on what you want.... Even then, there are several solutions, all of which will work....

Bob

[dyotat100]

100 fpe with a .30 means pushing pellets beyond the speed  of accuracy or shooting the 70 gr slugs that is just ok in the 1:26 barrel.  So far nothing shoots as good as the jsb at 100 yds.

Wonder why you didn't go with .357 and the JSB.

[oldpro]

 Lloyd if you can get your plenum up to 70cc minimum and open up that TP to .250 and your gains will be HUGE!!! And you will be able to drop the set pressure to gain back the lost head room of the added plenum size, well almost.

[lloyd-ss]

OK, seems like the natives are restless  , so the next step is to open the T-port up and work with the reg set pressure.  With the goal being 80-100 fpe in .30 cal, hopefully, the existing 45cc plenum will be adequate.  Right now there is 45cc regulated and 173 cc high pressure, so it is a balancing act of reg pressure vs volume vs shot count.

We could have opened the T-port up long ago but I think we learned something about the compromises of a small T-port.  Still, if the goal were say, 60-80 fpe, we'd be in very good shape.  So with the larger T-port and long barrel (actually, ending up at about 25"), and lower reg pressure, we should be able to work well within the 80-100 fpe range.  This is really a learning exercise about the impact of changing the variables in the Marauder rifle.  I think we are learning a lot, and that is the whole point.   Personally, I feel like if don't learn something new every day, I might as well have not gotten out of bed, LOL. 


BTW, the .357 never was a consideration, this was always a test for .30 cal platform with Huma reg and Cothran valve.  The .357 would have to be a whole 'nother project.

Lloyd

[lloyd-ss]

The installation of the new .25" I.D. T-port is complete.  The hole in the breech block and main tube were both machined out from O.E.M .276 to a much larger .375 dia.  The barrel port was also opened up to an oblong shape of .278 x .220, and the airway in the end of the bolt was also enlarged.  A matching plastic T-port (.375 O.D. x .250 I.D.) was made, but it was short lived. 



The T-port didn't last a single shot, and its replacement was made from 7075 aluminum and seaedl at both ends with o-rings.  That worked well.


After everything was working properly, and adjustment tweaks were made to the striker and the hammer spring,  I tried a shot string from a 3100 psi fill.  The results were good, IMHO.
With a 2360psi reg pressure, there were 11 shots within a 3% velocity spread, 938 fps to 910 fps (the shots in yellow).


But the really interesting thing was how powerful the remaining shots were in the low pressures after the regulator went below its setpoint.

You can see that even down at 1700 psi, it is still making over 830 fps and 78fpe with the JSB 50 gn pellets.  That gives a lot of tuning options just by changing the regulator set pressure.
BTW, the BBT 71gn bullet, at 2330psi,  went 885 fps to make 123fpe.  This is all still with the 45cc plenum.
Lloyd

[oldpro]

I can't believe your making that much FPE with such a tiny plenum. But the numbers don't lie. I still believe you'll see even more power with a larger plenum and be able to drop set pressure and gain back headroom you lost with bigger plenum. I personally think your doing excellent work my friend. Two thumbs up.

[Sfttailrdr46]

That is great results to say the least . Is the barrel still 27" or did you shorten it yet

[lloyd-ss]

Yes, this is still the 27" barrel, I will be shortening it next.
The plenum volume is 45.1cc total :  29.5 cc in the plenum that came with the Huma, 9.2 cc in the hogged-out gauge block, and 6.4 cc within the Cothran valve itself.   The remaining High Pressure area of the tube is 173 ccs.
Lloyd

[rsterne]

Lloyd is compensating for the smallish plenum with a bit more pressure.... Now that he has a larger transfer port, as he said, he can drop the setpoint pressure and still have good power.... I think the guys that shoot that TJ's 26" twist barrel in benchrest set up for just under 900 fps.... IIRC, 880ish with the 45 gr. JSBs is a particularly accurate setup.... I'm pretty sure that's what Tim @ Mac1 used to win the EBR in 2014....

Lloyd, I had exactly the same problem using a Teflon transfer port with a 1/4" ID and 3/8" OD with the end machined down to fit inside the O-ring buried in the Cothran MRod valve....



I took a few shots though, with it leaking badly, until the velocity tanked.... It looks like the pressure got between the O-ring and the plastic and then the next shot(s) with the pressure up on the outside of the TP collapsed it.... Nice souvenir, tho....

I think I would try a setpoint of around 2100 psi.... you should be able to get 2 mags. at that pressure, at very close to 900 fps....

Bob

[Motorhead]

Lol ... come on Lloyd, we already covered the requirements of a thin wall transfer tube !!!
You have Peek, use it man while loosing the o-rings ... JMO

Otherwise great R&D getting a handle on all that happening 

[lloyd-ss]

Bob, The soft material along with the o-rings has some limitations.  I am excited to see what happens after I shorten the barrel and adjust the reg pressure.

Scott,  I have a complete collection of 1mm C.S. o-rings, and T-ports seem to be a great use for them, LOL.     Actually, this set-up had a slight misalignment such that the T-Port didn't set exactly square.  I think the o-ring helped compensate for that misalignment.

Travis, I don't know if it making any difference, but the hammer hits a damper pad on the back of the valve which might possibly increase the dwell and kill the hammer bounce.  Could possibly be utilizing a tiny bit more air out of the plenum that way.

Lloyd

[lloyd-ss]

The deed has been done, and the 27-1/2" long barrel has been shortened to 24".  This puts the muzzle back about 2-1/4"  inside the shroud.  That is enough room for the air stripper, a spring, and one baffle.  The impartial observer on the other side of the wall said the noise was no louder than usual (when a muffler was used), so that is a good thing.

I went ahead and took 5 shots, without changing any of the settings from the shots taken when the barrel was 27-1/2" long.  Those are in reply #344, above.  After I took the shots I went back and found the piece of barrel that I had cut off, just to be positive I had shortened it by 2-1/2". 
After I filled the reservoir, the plenum pressure for the first shot was 2375 psi, and for the remaining 4 shots it settled into  2333 +/- 2 psi.
The repeatability was nice. 50 gn JSB's.
Shot 1  935 fps   (2375 psi)
Shot 2  922 fps   (2333 psi)
Shot 3  922 fps   (2333 psi)
Shot 4  919 fps   (2333 psi)
Shot 5  921 fps   (2333 psi)
(I stopped shooting after 5 shots.  Not sure how many shots at this power level are available.)

Looking at the data in Reply # 344, it seems pretty clear that for this pressure, dwell, and pellet weight, the 27-1/2" long barrel was definitely in the region of diminishing returns.  Hardly a nickel's worth of difference between the 27-1/2" barrel, and the 24".  That is, at 2333 psi.
Lloyd

[oldpro]

 Lloyd Im sure glad you cut that barrel down to 24" I cant tell you how many rifles Ive built but its A LOT! and what I have found after all the different builds on many calibers is 24" is the sweet spot. Longer is only marginally better. Sure is nice to see others have the same result. You still need a bigger plenum LOL. Keep up the fine work my friend.

[jayman07770]

I'm so glad you guys a sharing this information. Can't wait till a drop in valves with matching transfer port are available from someone. I'm happy with the current results that Lloyd's getting right now. It's more than enough for my small game hunting.

[oldpro]

You cant have a drop in T-port to match valve without machining breech its the one drawback the marauder has but its not hard to open up the breech but its a one way trip no going back.

[lloyd-ss]

You cant have a drop in T-port to match valve without machining breech its the one drawback the marauder has but its not hard to open up the breech but its a one way trip no going back.

And the hole in the air tube, too, to fit the larger O.D. T-Port.  Travis, I agree, the small O.D. of the factory T-port seems to be the choke point for the M-Rod.  That's why I was working so hard to get everything I could out of the factory O.D. port, and .218 was the max hole size for that port.  Opening the port up to a .250 hole (which required machining the breech block and the air tube) made a significant difference, just like we all knew it would, but now we have a better idea of what the ceiling is with the smaller O.D. factory port.
Lloyd

[jayman07770]

I'll settle for machine the breech and air tube any day for these gains.

Oldpro or Lloyd , let us know if you're planning to make a .30 valve and TP kit aviailble. I would definitely be interested.

[rsterne]

You need to replace the MRod valve to take advantage of a transfer port larger than what you can fit into the standard tube and receiver.... The exhaust port hole in the Crosman valve is so close to the valve seat, going larger than 3/16" threatens to break through into the back of the valve.... I think 0.203" (13/64") is the largest I have heard anyone managing without destroying the stock valve.... The Cothran valve, and the WAR valve, and any others that can have larger ports than that, have the valve seat moved forward.... In other words, to have any reason for a larger than stock OD transfer port you must replace the valve also.... Just wanted to clarify that....

To use the full potential of a Cothran valve (which has a 1/4" exhaust port), you must make a larger OD transfer port, which requires drilling out both the air tube and receiver to accept it....

Bob

[jayman07770]

Drilling the air tube and breech doesn't sound to complicated. Besides drilling the barrel transfer port. Do we need to do any other machining to the barrel to insure a good seal. The barrel I have is only machine to match the stock OD TP seal.

[rsterne]

Yes, if you increase the transfer port, you also need to machine the barrel to mate to it properly without leaking.... Generally, that means a larger flat on the barrel to seal against the larger OD transfer port.... One of the limits to the caliber is the thickness of the barrel wall.... If the caliber is too large, with a 1/2" OD barrel (to fit the MRod receiver).... it is difficult to get a larger enough diameter flat without hitting the bore.... A .30 cal is OK on an MRod if you take care.... a .357 cal is getting VERY difficult to do without ruining the barrel.... I'm not saying you can't do it, but it ain't easy....

If someone produces a .30 cal kit for an MRod, it would normally have a machined barrel, bolt, transfer port, magazine, and valve, as necessary parts to get the performance you want.... That would leave you to drill out the air tube and receiver for the larger transfer port OD....

Bob