Half shroud on a Marauder for better accuracy

My Marauder FT .177 with half shroud and 5 out of 7 baffles. In theory, this might also have a beneficial side effect of placing the mass of the rear shroud bushing at the first point of the golden ratio of the free barrel length where it would have the most dampening effect on barrel harmonics. I am right now at 7.5 inches from the crown. If I put one more baffle back in it would be at the ideal 6.8 inches. The inverse of 1.618 times the free barrel length of 17.5 inches.

.



.

5 shot groups from each half shroud Marauder. 12 yards benched indoors.

.

FT .177 top with JSB heavies at 865 FPS. The max velocity spread is excellent at 6 fps for at least 60 shots. It was still on the reg at 2,000 psi when I refilled. And plenty of power. It will easily do over 900 FPS with those pellets for 19 FPE. I have the spring at the minimum and the striker turned in 8.5 turns to get down to 865 FPS. I may turn the .177 down a bit more to find a tighter group when I get out to longer range so I can see what it is really doing.

.

.25 cal bottom with JSB 25.4 Kings 825-835-825 FPS. Truly one hole. Can't wait to get outside.

.



.

I have a sound meter somewhere but didn't do any noise testing before and after. I may dig it up at some point to see what it measures at now. Just on a guess I fail to see how any/ much sound actually comes through the barrel wall of the missing portion of the shroud. The bigger change I made was in removing two of the 7 baffles to shorten up some of the immense overall length of the Marauders. The report is generally still quieter to the shooter's ear than the valve strike and air cylinder ping. Anyone that wants to try tightening down the barrel to the band with rubber or foam can still experiment with that in addition to the half shroud.

This machine shop did really nice work. If they could set up and do 10-20 at a time it would be much cheaper. Let's give this thread a couple weeks to see if any kind of group buy might develop. There was someone else selling these half shrouds complete from their own tube stock for $60. The one on my .25 is from them but the project sat around for two years before I put it together so I don't remember who it was. I had a big problem with that one anyway. The front cap screwed right on but the rear threaded bushing has a slip fit shoulder that has to slide into the id of the tube first before the threads start and apparently when they cut the threads, they used a technique that ended up knurling the crest of the threads out higher than the original id of the tube. So I was in effect, forced to screw the tube's threads over the flat shoulder of the bushing before even engaging the male threads. Which of course distorted the hell out of them. I barely got it on and I might have to slice it if I ever want it off. Fortunately my math was good on the installation position of bushing and it appears to shoot great so I shouldn't have to take it off for anything since I can still unscrew the end cap and dump out the guts for cleaning.. I mentioned this to my new machinist to make sure it didn't happen again and he did a perfect job as seen in the photo.

Got my 25 yard shooting range somewhat set up so I could get a better idea of the accuracy beyond the 12 yards I can do in my attic. The .25 is shooting through one hole as it should at 25 yards. The .177 with Crosman barrel needs to get better. I will polish the bore when I take it apart to install the hammer debounce device I ordered. I'm surprised that it is burping even with the regulator. I could also find a better tune since I have the striker wound all of the way back long. I will also play with adding some set screws or spring plungers to the band.

.

The tripod is working out very nicely for a hunter/ sitting on a 5 gallon bucket position and groups almost as well as the wobbly bench (folding table) I have been using. I'm making a more solid bench today.

.



.

The best group of three from the .177. It got late at 9:30 and the mag was empty so I didn't want to take a chance on disturbing the neighbors (even though the 177 makes more noise from the hammer than the report at my position). I live on a busy street and just hold my sight picture until a car passes by out front to remain fairly stealth. The HN Hunters fit really tight and made a terrible group. Crosman Pirahnas barely fit in any of my 177's. Don't bother... JSB Heavies for now. And ordered some Monster new design to try since this rifle makes plenty of energy to get them to 20 fpe.



.



.

Ten shots with the 177 at 26 yards and down 6 degrees from the tripod out my upstairs window. I didn't plumb the target this time as I usually do which lets you cheat to find level with the cross hairs and didn't pay close enough attention to the canting after cocking each time. Still learning all of this new gear.

.



.

The rear of the shroud threads onto a bushing which is held wherever you want it as it slides along the barrel until you tighten the set screw. From the factory it is placed right up against the breech block. I may end up tapping the barrel band for nylon set screws or spring plungers anyway. Which would probably work better with the full length shroud which I still have. There is quite a bit of barrel droop even with the thick 177 barrel walls. Put a Marauder horizontal for a day and see where it is in relation to the circumference of the band. And then leave it vertical for a day and check again. There is probably 1 mm difference. Barrel droop yields increased barrel whip and harmonics.

I tried the JSB monster redesign 13.3gn in the Mrod FT 177. The skirts are much larger than any other 177 pellets. I had to poke them with a pencil to get the skirt to go into the mag. With everything cranked I could only get them to 775 fps for 17.7 fpe. I'm now finding that barrel length and air are limiting the JSB 10.3gn heavies to 880 with the hammer debounce device installed since it took much adjustment to start making any change. The HDD did completeIy cure an audible burp. I ended up with the spring in 3 turns from min and the striker in 5.5 turns with the port adjust pintle removed and sealed by the locking cap to get them back down to 860 fps and 16.9 fpe where the adjustments were coming into good control and the hammer strike was less violent. Shooting over the chrony indoors at 12 yards doesn't give much feedback on group size but it seemed like they opened up at 845 so I tuned it back up. Turning the striker in to reduce the power also begins to give a sensation of a reduced lock time from when you pull the trigger. The picatinny rail made it easy to steal the 10-50x60 ED Hawke off of my Creedmoor for a trial. It is fun drilling paper at 40 power through really clean glass. Great glass but the IR has much increased parallax compared to the black lines on the reticle and the black is so fine that you can barely see it at 10x so you pretty much always end up dialing in some red and using more magnification than you want for quick acquisition. I think green would be better for my eyes but it doesn't offer it. Maybe it is particular to me but both my Airmax and the ED have the minimum parallax at a setting that just out of focus toward the long side of the adjustment. I cheated the Airmax by compromising the diopter between best image focus vs minimum parallax at slight expense of reticle sharpness. Which again displaces the fuzzier red from the black when moving your head. The 50x scope reticle is so fine that I had to favor the best reticle sharpness. The black is only moving a tenth of a mil with extreme head nodding so not a big deal if you center in the exit pupil well. But the red shifts plus and minus half a mil. And it could use a bit more than 45 MOA of total adjustment. I'm barely making it up to 12 yards from a 2.06 inch scope height.

My luck isn't so good with my new Marauders.

The .22 FT Lothar Walther with Hill tight barrel band and Hawke Airmax 32x 50mm scope doesn't like HN 14.66 FTT that I had on hand at all and the regulator design places it right against the valve with minimal plenum and a lowish pressure so 22 fpe is all you can get. Crosman Hollow points were somewhat worse. JSB 18.1gn heavies are too slow at 750 fps and don't group quite as well as the 15.89's

.



.

It likes the JSB 15.89 slightly better but is still looking like 2 MOA at 12 yards indoors in my attic. I haven't had the chance to shoot them at 25 yards yet (still snowing every day at the end of April! What the hell.) Maybe tomorrow.

.



.

It's frustrating because my half shroud Marauder .25 with Green Mountain barrel and debounce device shoots through one hole at 25 yards. JSB 25.4 at 840 fps 5 shots each through a cheap Leapers 32x 56mm scope. So this tells me the Marauder platform can group but this LW barrel on the .22 isn't producing. I have a full selection of .22 pellets coming in the mail (Pyramyd took 10 days to pick the order) so whenever I get those I am going to document all of them to Crosman with the Airmax 32x 50mm and again with the Hawke 50x 60mm ED off of my Creedmoor to see if they will swap barrels.

.



.



.

The .22 Lothar FT has the Hill tight barrel band on it. It is a nice piece. For the .177 I am playing with just sliding a piece of foam in the band to push the shroud up against the top of the factory band.