.25 cal Carbine Based on MRod Tube - Includes Build Details from Reply #33

[rsterne]

My tube does not have a hole for a gauge.... hence the need for a vent hole instead....

Bob

[HunterWhite]

My tube does not have a hole for a gauge.... hence the need for a vent hole instead....

Bob

I just wanted to suggest that you get the reg before drilling the vent. You might be able to place it where you can use a gauge if you want to later on.
It would be in the same area.
Where I have just a vent it is tiny, like. 030 (.75 mm) or .040 ( 1mm)



Hunter

[rsterne]

Yep, thanks.... Good idea to use the original gauge location.... 

Bob

[jackssmirkingrevenge]

I can vouch for Huma regulators, I fitted one to an FX Streamline and it was quite a dramatic improvement over the stock regulator, set at 130 bar standard deviation dropped below 1%.

[rsterne]

Thanks, Jack....

BTW, by Canadian law, the barrel on any firearm cannot be modified to less than 18", so that will be what my Carbine will be.... 

Bob

[jackssmirkingrevenge]

I understand erring on the side of caution when it comes to legal matters, but it does seem like a grey area to me.  One definitely sees non-restricted powder burners with barrel lengths well below 18 inches on this side of the border, the caveat seems to be that its a factory barrel and not something one has cut down, that it is a rimfire or manually operated action if centerfire, and the overall length remains over 26 inches.  My understanding is that I could take say a Ruger 10/22 that left the factory with an 18.5 inch barrel, install a 12.5 inch factory barrel and still be within legal limits.  I certainly wouldn't want to risk finding out the contrary in court but within reason shouldn't the same apply to a bolt action air rifle too?

[rsterne]

The rule for semi-auto center-fire is now 18.5" minimum (new rule, I believe, even for new or existing rifles) otherwise they are Restricted.... If you are "modifying" any Firearm barrel and go less than 18", it becomes Prohibited (been that way for years), but there has always been the argument about "I didn't modify the barrel, I bought it that way (maybe even from a different model)".... Like you say, do you really want to test that?.... Back when you had to register Non-Restricted rifles, I registered my current Grouse Gun, based on a Disco tube (shortened) and using a .22 cal, 14.6" Crosman barrel, with a homemade breech, and got a registration for it.... 

Bob

[jackssmirkingrevenge]

Like you say, do you really want to test that?

Alas that's what it boils down to. I was going to say one might as exploit another quirk in our laws and go for something in a much larger caliber but under 500 fps so one no longer has to be concerned about length, but of of course you've already figured that out a long time ago

https://airgunforum.ca/forums/viewtopic.php?t=38387

[3crows]

If I follow the question if there remains a question? The XXL Huma Plenum is the .25/.30 plenum. The O-ring in the #1258 bag would go in the end of the XXL plenum #1258 and seal either to the gage block manifold face (note O-ring removed from the forward end of the gage block manifold for venting of the regulator through the gage hole) or the plenum #1258 could seal directly to the valve inlet face. Venting could still occur through the no longer used gage hole in the reservoir tube or if you had a tube with no gage hole then a vent hole would have to be drilled.



There is a tiny vent hole on the Huma regulator, it is barely visible if you look close. The standard plenum with integral gage hole is #1259, not used in this build.

[rsterne]

I am using a custom aluminum tube, with no gauge hole, so I need to drill a vent hole in the tube.... I will probably do a small (1/32") hole where the gauge port would be....

I checked the volume of the plenum with gauge port, and it's only about 16cc, so including the valve about 20cc.... A .25 plenum with stock gauge port is larger, and can be made even bigger by hogging out the gauge block.... However, the XXL (no gauge port) is 31cc, so including the valve 35cc.... Since I plan 35-40 FPE, that is nearly the 1cc/FPE I strive for in my regulated PCPs.... I used Lloyd's spreadsheet to run a comparison between 20cc and 35cc.... and the smaller plenum requires a 100 psi higher setpoint to equal the power of the larger one, and has nearly twice the pressure drop during a shot.... Net result is that even with the smaller HP volume, the larger plenum is the winner on shot count....

Bob

[Motorhead]

When i place holes for reg venting I use a #60 drill ( .040" )  Be sure to deburr inside of tube where drilled.

[Wildman73]

I am thinking of replacing my .22 cal Grouse Gun with a .25 cal, and of course it will need more air.... I can use a 2024-T3 aluminum tube that is 1.25" OD x 0.095" wall (like a Marauder), and install a regulator inside, and get about the same number of shots I have now (16 is desired, ie 2 mags.), at the same gun weight.... My current gun fills to 2000 psi and has a 14.5" barrel and uses a 2260 tube, and looks like this....



Imagine that with an MRod diameter tube, and about the same length .25 cal barrel, and you get the idea.... Velocity with a 25.4 gr. JSB to be between 750-850 fps....Now for the question....

It would make sense for me to use an in-tube regulator, such as the Huma... I want a plenum volume between the regulator and valve of 28-40 cc, and I am wondering what is available?.... What brand of regulator would you recommend?.... I am particularly interested in what Motorhead has to say about this.... What setpoint pressure would you recommend for 800 fps with a 14-18" barrel?....

Bob

Man that is awesome as it is! What power now? Fpe with the 22?

[rsterne]

It is 20+ FPE (18.4 gr. @ 700-725 fps).... Does a great job on head shots, me not so much any more.... My son's rifle is a Hatsan AT-44 Long in .25 cal at 70 FPE, and when he hits, they just drop.... However, it's a long heavy rifle, and the 34 gr. King Heavies just blow through, so I figure half way in between, .25 cal holes, with the size and weight of my current Carbine, should be about perfect!....

Bob

[rsterne]

Here is a photo of the completed .25 MRod Carbine.... I 3D printed the stock from CF-PETG.... Scope is 3-9 x 32 Leapers I had kicking around, focuses down to 6 yds....



Plateau (max.) velocity regulated at 2000 psi is 935 fps, I will be tuning for 900-915 fps (~47 FPE) with 25.4 gr. JSB Kings.... LOA is 33" with an 18.5" barrel, and it weighs 6.25 lbs. with the scope.... I am likely going to move this thread to the Workshop and add some build details....

Bob

[rsterne]

OK, so now that I have finished this Carbine build, I thought I would write a short summary with a few photos.... First of all, MANY THANKS to my friend Lloyd Sikes for collecting and supplying the parts I could not source in Canada.... He got a bunch of stuff from Crosman, and a piece of 2024-T3 Aluminum tubing in 1.25" OD x 0.095" wall for the main reservoir tube.... Once again, I am in his debt.... Merry Christmas, buddy!....

The first thing I built was the rear plug incorporating the SSG.... The hammer spring is 0.48" OD x 3.0" long x 0.055" wire (one wire size down from an MRod spring and 1/2" longer).... It has about 0.4" of preload, which was a complete guess.... The preload is adjusted by the two nuts locked together on the back of the guide rod.... The drilled out 5/8" NF bolt adjusts the gap between the end of the spring guide and the inside of the hammer.... The rear plug has two tapped 4-40 holes in the top for the rear breech screws, and one 8-32 in the bottom for the back of the PRod trigger group, plus 2 slots milled in the side to vent the area behind the hammer to prevent a vacuum there on firing (it really messes up the power and shot-to-shot consistency without venting).... The hammer is solid steel and weighs 105 gr. and drilled 1/2" by 1" deep for the spring.... I thought I might use a lighter one but it wasn't necessary (more on that later)....

The PRod trigger group has the top reprofiled to fit the larger MRod tube.... There is a second reason to machine the top, as the MRod tube is thicker wall, and without lowering it, the sear will not stick far enough through the tube to safely engage the hammer.... So, lowering the top side edges 0.030" does both jobs, you can see where the metal was removed in the photo.... Note the center of the top curve was not touched by the 1.25" ball mill....



You can click on the photos to enlarge them.... I will be doing a new Reply for each step, to be continued below....

Bob

[rsterne]

The next step was all the internals, but to be truthful, most of them I had lying around.... Lloyd sent me a Huma XXL regulator he had, already set to 2000 psi, which was about what I figured I needed.... I installed it like that, and it was perfect!.... The valve was one I had made as an experiment to see what could be done to a stock MRod valve to make it breathe better.... Those mods are covered in this thread....  https://www.gatewaytoairguns.org/GTA/index.php?topic=44258.0  and also here.... https://www.gatewaytoairguns.org/GTA/index.php?topic=111416.0

Enlarged Exhaust Port, diameter 0.200"





Conical Valve Spring



Mod to Inlet End for Spring



I also shortened the back end of the valve body by 0.075" and the valve stem by 0.150" to increase the hammer stroke.... The Huma regulator sits against the front of the valve and seals to it with an O-ring.... Since I did not have a hole for the gauge, I had to drill a small vent hole to vent the regulator....  I don't have a photo of the front plug, but it is just a simple aluminum plug turned to fit the tube ID, drilled and tapped for three 10-32 mounting screws like the valve, plus 1/8" NPT tapped holes, one in right side for the gauge and one in the front for the Foster fill fitting.... Here is a photo of the assembled pressure reservoir....



Again, click to enlarge.... The primary side contains 9 CI (145 cc) of HPA and can safely handle 3200 psi according to Lloyds pressure reservoir spreadsheet.... The regulator is set at 2000 psi, and has a 35cc plenum, plus about another 5 cc in the valve.... If there was no regulator, the reservoir would hold almost 200 cc, just a bit smaller than a stock MROd....

Bob

[rsterne]

The next step was to make some minor modifications to the Marauder breech, to accommodate the larger transfer port... I chambered the barrel with one of my custom chambering reamers, so that the pellet when chambered was just ahead of the front of the barrel port.... The shallow taper cut by the reamer meant that the back of the head was fully engraved by the rifling, while the skirt was just starting to engage the lands.... The transfer port was 0.200" in diameter, with a 5/16" flat for the Delrin port to seal against.... The breech was bored through to 5/16" to match as well....

The barrel was shortened to 18.5" and crowned with an 11 deg. "target crown".... I made three collars to fit over the barrel, fastened with 8-32 setscrews, one against the front of the breech, one at the muzzle, and one halfway between.... The front two were grooved to fit a 3/4" OD x 0.102" thick 70D O-ring which was a slide fit inside the 1mm wall Carbon Fibre tubing I used as a shroud to help stiffen the barrel.... The shroud is then held in the middle by a clamp that is part of the 3D printed forestock (more on that later)....





I modified the design I had for a 3D printed stock for a 2260/Disco size tube to fit the larger MRod tube, and also raised the comb line level with the top of the larger tube to locate my cheek in line with the scope.... In addition, I created a 3D printed forestock that slides over the MRod tube, and fits under the shroud tube.... It has a small clamp at the back just in front of the trigger group to snug it onto the main tube, and at the front of it, it has a cradle to fit the shroud and a beefy clamp using four 8-32 SHCSs to clamp the shroud rigidly to it.... Here are two photos, one showing the top and the other the bottom of the parts....





The adjustable buttplate is shown in the middle of its adjustment range, it can be raised flush with the flat on the bottom of the stock for shooting off sandbags on a bench....

Bob

[rsterne]

First assembly was successful, except for a small leak.... That turned out to be the gauge itself, the air was escaping through the vent hole in the back of it!.... Replacing the gauge gave me a sealed PCP.... I did the initial shooting tethered to an SCBA tank that had 2500 psi in it.... As I said, the regulator was set to 2000 psi, although with no second gauge I had no was to be sure of that.... Shooting a string to see where the velocity dropped provided that answer, and it was indeed 2000 psi on the small gauge on the side of the gun.... I can't believe how close my initial tune was to what I ended up with.... As I do with any regulated PCP, the first thing I do is shoot a few shots at various SSG Gap settings to see where the plateau is, and what is the maximum velocity.... This ideally also shows me where the "knee" of the curve is, and lets me know if I have enough adjustment on the gap to arrive at the tune I want.... In this case, the regulator setpoint of 2000 psi, the hammer spring I chose (along with its preload) gave me a knee on the curve at about 2-2.5 turns of gap on the SSG.... absolutely ideal.... Here is a chart of that initial testing, using 25.3 gr. JSB Kings....



If the SSG gap (between the end of the spring guide and the hammer) is too large, the hammer strike can vary depending on gun position (muzzle up or down), but with the 20 TPI of the adjusting bolt, 2 turns is just 0.10", just about perfect.... So, I didn't need to change anything, just get on with the fine tuning.... Also, the power looked to be more than I needed, so that was a relief.... Now all that remained was to tune for the best balance of power and shot count within that narrow area between 2-2.5 turns of gap on the knee of the curve....

Bob

[rsterne]

Today was one of the most pleasing and rewarding I have had in a long time.... I knew my new Carbine was a winner, right out of the box, but I was shocked how little I had to do to fine tune it to far exceed my hopes and expectations when I started this design and build.... I tried 2.0, 2.25, and 2.5 turns of gap on the SSG, and I discovered that the tuning was VERY sensitive within this range, as you might expect with it being on the knee of the curve.... At 2.2 turns of gap with the JSB Kings and a 3000 psi fill, I had an average velocity over the first 16 shots (above the 2000 psi regulator setpoint) of 907 fps, with an ES of about 24 fps (2.6%).... Below the setpoint the velocity climbed slightly before falling below 900 fps at shot 28.... This is the red curve on the chart below.... I decided to reduce the gap just a bit to 2.1 turns (0.105"), filled to 3000 psi and shot another string.... The first 3 shots were all 919 fps, and after 16 shots (2 mags) the average velocity was 919 fps (47.5 FPE) , with an ES of just 9 fps (1%).... That is just about perfect for a regulated PCP.... Below 2000 psi, the next 8 shots also fell within the same 1% ES, so I shot 4 more shots to see the velocity decline.... By shot 28, it was still 894 fps (the black line on the chart).... WOW !!!! .... This meant I have 3 mags. (24 shots) within a 1% ES, ending at 1600 psi (400 psi below the setpoint).... I have never had such a flat, extended shot string on a regulated PCP before.... Dotted lines are below the 2000 psi setpoint....



I wanted to find out what it did with a couple of other pellets, the 26.6 gr. JSB Hades HP, and the 33.7 gr. JSB King Heavy (Mk. II).... I didn't change the tune from my 2.1 T gap on the SSG, and those strings are shown in blue (Hades) and green (Heavy) on the chart as well.... Both had a greater shot-to-shot variation, the Hades was 17 fps (1.9%) on a 903 average (48.1 FPE) and the Heavy had an ES of 18 fps (2.2%) on an average of 828 fps (51.3 FPE).... All of these are perfectly acceptable for anything up to about 100 yards, and the gun will be primarily used within 50 yards....

The gun is fitted with a Leapers 3-9 x 32 Scope which I had lying around, and it should be perfect for Grouse and Hares, and focuses down to under 6 yards.... When I shoulder the gun, I'm looking right through the scope, perfectly centered, so the ergonomics for me are ideal.... The gun weighs just 6.25 lbs. complete with the scope, and is just 33" long.... My goal was at least 2 mags (16 shots) at 40 FPE.... What I achieved was 3 mags (24 shots) at 47.5 FPE with the 25 gr. JSB Kings, which we all know is a very accurate pellet.... Grouse, lookout this fall !!!!

Bob

[Waldo1]

Always a pleasure to read up on your builds, Bob. Your grasp of the nuances involved is impressive and your documentation is first-rate. It inspires me to get up off my @ss and get out into the garage.