The AirGun Guild

A friendly place to share one's interest in all things airguns.

So looking back at my max velocity, 10 shot bell curve:

887
889
890
896
892
894
893
891
893
890

So I'm averaging just over 891.5 fps with the stock 17.7" barrel. My question is if anyone has any idea what sort of fps increase I might get with a 28" TJ barrel. I am wanting to shoot 29 or 33.5 grain NSA slugs as fast as possible over the course of 10 shots. My hope is that I might gain, at minimum, 50 fps for a 10" barrel increase. But I could see it being more since I'm currently pushing 45 fpe in a large caliber and short barrel.

With a 28" barrel and the 33.5 gr. slugs.... Lloyd's Internal Ballistics Calculator says about 840 fps (52 FPE)…. However, by leaning on the hammer a bit, with the longer barrel you should be able to gain even more and still maintain a bell curve.... because the longer barrel can stand more dwell.... I don't think 60 FPE with the slugs is out of reach with a 28" barrel....

Bob

That's exactly what I'm hoping for. If I could get 10 shots @ 60 fpe, I would be so happy. Even if it was just 5 tight shots @ 60 fpe, I would be okay. Now I just gotta find someone who will still machine a TJ barrel for the Flash. JSAR is booked up, Jim with WAR has closed down for the time being.

Here's another data point... I grabbed some 33.95 MKII and tuned the gun to those for 10 shots (one magazine). Here's my shot string:

793
795
801
798
799
798
799
793
797
798

1% spread with an average of 797 fps (average of 47.3 fpe). I'd love to see what this thing can do with a 28" barrel.

Awesome, thank you so much Bob. I was finally able to reach someone at TJ for the barrel. They have a 28" x .500" OD barrel that is .250" groove, .243" bore, 1 in 14" twist. I'm assuming this is the barrel I need for the Nielsen slugs. Just wanted to double check before I send the money. My plan is to sleeve it with this: https://www.clearwatercomposites.com/product/1-2-x-3-4-carbon-fiber-tube/

TJ's have two different .25 cal barrels (maybe 3) of that diameter.... They have the 14" twist ACP barrel, a 22" twist airgun barrel, and I understand they also have an 18" twist, but I don't know if that is available to anyone, or proprietary.... Which twist rate you want would depend on how long a slug you wanted to shoot.... I have used the NOE 55 gr. boattail in he 14" twist very successfully.... but that may be a bit fast for lighter, shorter bullets.... For Nick's slugs, you would be best to ask him what twist to get....

Here is an alternative source for your CF tubing....

https://dragonplate.com/carbon-fiber-roll-wrapped-twill-tube-05-id-x-72-gloss-finish

They are less expensive, but you can only get the 1/8" wall tubing in 72" lengths.... The best way to sleeve the barrel is to insert the CF tube into the breech about 1.5" (2 diameters) to support it properly....

Bob

So, if I'm having someone machine the barrel to a Hatsan Flash, I don't think there's room to sleeve the barrel into the block. I was pretty much banking on the carbon fiber butting up against the face of the block and that being stiff enough.

Edit: The other option is to get the barrel in 9/16" and only do a 24" barrel. That's only 6" longer than the stock barrel and the stock barrel is also 9/16". I can't see another 6" being a huge problem. I'd still sleeve in a thin CF tube, not that it would add much. But then I know that it's basically like a longer factory barrel. Similar to a factory AT44 Long barrel.

I am not familiar with the Flash.... Does it have a barrel band, or is the barrel free floating.... If free floating, adding a sleeve will stiffen the barrel itself, but if not inserted some distance into the breech, it will tend to wave around, bending at the front of the breech.... If, on the other hand, there is a barrel band to prevent that, then sleeving it will help a lot....

Bob

So, the stock Flash here in the US has the QE shroud system on it. The OD of the shroud is .75" as it passes through the barrel band. However, the barrel band does not tension the shroud at all; it's simply there as an overtravel stop sort of thing, in case the barrel gets bumped. What I have done, however, is pull the QE shroud off, exposing the 9/16" barrel, so I could use a DonnyFL Ronin. I then slid four 7/16" ID by 3/4" OD rubber grommets on the barrel. By stretching the rubber grommets 1/8" to get over the barrel, it makes them a very snug (required silicone grease) fit in the barrel band. So now the barrel is attached at the breech, then very snugly held by the barrel band/grommet combo. Accuracy is still ridiculous (sub-1/4" groups @ 33 yards) and I have to actually wrap my hand around the barrel/airtube and squeeze to get the barrel to move at all. TBH, I think I could do a 28" TJ barrel in 9/16" and it would be plenty stiff and secure with just the grommet solution. Obviously a hard knock might throw things off, but the rubber grommets do a great job of soaking up any vibration/knocks and hold the barrel very secure.

If you decide to use the 3/4" OD carbon sleeve, it sounds like it could be supported in the barrel band.... You could try machining a recess for an O-ring groove in the barrel band, to hold a compressed O-ring on the shroud.... or even try 3 setscrews.... I like the idea of stiffening the barrel with a 3/4" CF tube, and locating it with the barrel band.... that will eliminate the barrel waving around where it enters the breech.... You could go back to your idea of a 28" barrel with the CF sleeve.... That would greatly stiffen it beyond the barrel band....

Bob

I'd have to measure and see what the ID of the barrel band is to see what type of o-ring would be needed to fit the 3/4" carbon sleeve. I also don't have any way to machine a groove in the barrel band, and it is plastic, if that matters. I'm wondering if the 9/16" barrel with the .633" OD CF sleeve would be a good option. That would make the grommets stretch an additional 1/16" (total of 3/16") to fit over the barrel, providing an even snugger fit than what I have now.

Edit: I wasn't paying close enough attention to your last sentence. I don't know exactly how far forward of the breech the barrel band sits, but I'd imagine it's somewhere around 4"-6", with about 3" or so in the breech. So on a 28" barrel we could potentially have ~21" sticking out past the barrel band. If you think that the 3/4" CF tube is necessary to stiffen that long of a barrel, then I'll just have to figure something out.

Okay, so I pulled out the tape measure so we have actual numbers. The stock barrel is 17.7" long with the portion sticking out of the breech 15 11/16" long. Meaning that 2.0125" is inside the breech. The barrel band starts at 5.75" from the breech and is 1" long, ending at 6.75" past the breech. Meaning that the barrel band starts at 7.7625" from the leading edge of the barrel and ends 1" later. So, on a 28" barrel, there would be 19.2375" sticking out past the close edge of the barrel band. Or 19.7375" if you want to measure from the middle of the barrel band.

The ID of the barrel band is .78" with the OD of the stock shroud .75". The wall thickness at the thinnest part of the barrel band is .121", just under 1/8". I don't know if that leaves enough material to cut a groove into.

The other thing I noticed is that there is ~1/32"-1/16" milled off the bottom of the stock shroud to give it a bit more clearance to the air tube. Rotating the stock shroud shows that there is maybe 1mm between the airtube and a full .75" OD shroud or barrel. I don't know that it's a big deal, but it might be.

So, I don't know if there is enough material to machine an o-ring groove in the barrel band nor do I have the skill/tools to do so. I could find someone to do it at a cost. I don't trust the wall thickness at the thinnest part of the plastic barrel band to hold a grub screw. I could tap one screw on either side of the barrel going up at a 45° angle to push the barrel up against the top of the barrel band I guess.

At this point I'm starting to really wonder what length of barrel past the barrel band you would trust a 9/16" barrel with a .633" OD CF sleeve to stay stiff with the grommet mod that I've done. Using a density of .284 lb/in3 (quick Google search on the density of chromoly steel) a 9/16" barrel that is 10" longer than stock only adds .568 lb to the weight of the barrel out front, when accounting for the .25" bore down the center. I don't think the extra weight of the barrel is the problem, so it must be the axial stiffness of the steel at that length. Am I on the right track?

Here is a picture of the grommet mod. Basically I have a full 1" of rubber sandwiching the barrel in the barrel band quite snugly.