Air Gun Home :: View topic

rsterne · Moderator

While waiting for our new barrels to arrive, I thought I'd do a practice run using a .308 cal PB barrel I had.... I cut off the .300 Win. Mag. chamber, ending up with a 21" usable section, and then I turned down the breech end to 1/2" OD to fit into the breech, and machined in a chamber and leade and a transfer port and O-ring groove.... Here's how it turned out....

I assembled it into the breech to check the functioning of the bolt, and the nose pushes a pellet just past the transfer port and then withdraws into the "J" slot so that it is just clear of the port and completely out of the airflow, just as designed.... I also machined out the groove in a .25 cal MRod single shot tray to take the .30 cal pellet.... It clips onto the end of the barrel which protrudes 0.060" into the magazine bay, the same way the magazine mounts....

By adjusting the diameter of the O-ring groove inside the barrel, you can customize how tight the O-ring grips the nose of the bolt.... I have set it up for a noticable drag, but the pellet slides through fine.... I'm sure it will seal perfectly as I've done this type of breech seal in the past with great results.... While outside the original design parameters of the gun, this barrel and loading tray will allow me to test cast .308" bullets up to 120 gr. in single shot mode (providing I cut the chamber deeper), just to satisfy my curiousity....

Bob

ZipSnipe · Veteran Member

Great job Bob, aren't breeches fun to make? i discovered while making mine that for future breech design, I will use small 1/8 inch plates on the side so that I can mill out the plates to test how it works , replace the plates as I mess up.

Uploaded with ImageShack.us

rsterne · Moderator

That'll work!....

Bob

rsterne · Moderator

Just a couple of pics of the finished stock.... I'm very pleased with the results....

It's a Boyd's Blaster in Forest Camo (green, brown, black).... Inletting done by Norm at Discos-R-Us....

I finished it with 7 coats of Watco Danish Oil, wet sanded (2 x 220, 2 x 400, 2 x 600) and then a final coat of Minwax....

Bob

rsterne · Moderator

I got the gun assembled with the short .308 PB barrel and fired a few test shots tonight.... First the photo.... I know, it's ugly with the barrel shorter than the tubes.... and parts still needing bluing.... but you can see the potential there....

The gun was pretty hard to cock, I'll have to take a look to see if there is anything binding.... However, I was still able to cock it at coil bind, and the good news is that is way too much preload, even at 3000 psi.... The first shot was the fastest, the gun was very loud, and the 46 gr. JSB pellet screamed through the Chrony at 971 fps which is 96.3 FPE.... Four shots later it was down to 920 and those 4 shots used 700 psi.... I backed the adjuster out 2 turns, and got 6 shots starting at 953 and ending at 918, using 750 psi.... Out 2 more turns, and I was starting to get a string.... Starting at 2900 psi (my tank needs filling) I got 10 shots starting at 914, peaking at 929, and ending at 886, and it used 800 psi.... Backed out 2 more turns, starting at 2900, I got 13 shots starting at 861, peaking at 906, and ending at 860 fps at 2000 psi.... That string averaged 880 fps (79 FPE) at an efficiency of nearly 1.1 FPE/CI.... At 8 turns out the usuable pressure range is from about 2600 down to 1600, returning 14-16 shots in the mid 800s.... and at 10 turns, the string starts in the high 700s, peaks in the low 800s, and returns, but this is with running stock Disco pressures starting at 2000 psi and ending at about 1300.... and the gun is still shooting 65 FPE, three times what a stock Disco does....

These are just preliminary tests, using upsized JSB 46 gr. pellets in a 21" barrel.... I just couldn't stand the suspense of waiting until the new barrels get here.... The additional barrel length should add roughly 5 FPE.... but I have already surpassed my goal of 88 FPE in .30 cal.... so tonight's testing was a resounding success.... BTW, I was running a 3/16" transfer port, I can go to 7/32" if I want to....

Bob

rsterne · Moderator

I changed the gun over to a rear cocker last night, and also enlarged the transfer port to 7/32".... Here is the hammer, which now weighs 82 gr. with the cocking knob....

The gun is now very easy to cock.... I put a small O-ring on the shaft which can be used to directly measure the valve lift.... You simply slide the O-ring up against the back of the RVA screw, cock the gun, fire it, and the O-ring slides back exactly the amount the valve opens and you can measure the space with calipers.... I did that for all RVA settings from coil bound to 12T out, for the first shot, and the last shot before the velocity fell more than 4% below the peak.... The smallest lift measured was just under 0.06", and the largest was 0.13".... In virtually every case where there was a proper bell-curve, the lift at the first shot was about 1/16".... and at the last shot was less than 1/8".... This is a very valuable piece of information, and while it won't be the same for every gun, for this gun it indicates that limiting the valve lift to 0.15" (eg. by having the hammer hit the back of the valve at that point) would not have any affect.... That means I can lengthen the hammer stroke on this gun to a full 3/4" by grinding a bit more off the valve stem or counterboring the front of the hammer a bit more.... The fact that the valve lift was virtually the same regardless of pressure and preload.... once the gun was adjusted for a proper bell-curve.... was a bit of a revelation to me.... though in retrospect it makes perfect sense....

The larger transfer port increased the power a bit, made the shot strings more curved (and hence shortened them), and required about 3 turns less preload on the RVA for the same FPE level.... Possibly 1 turn of that is likely due to the slight increase in hammer weight with the longer guide and the cocking knob.... Here are the results of the testing, from coil bind to 12 turns out, by which time the fill pressure required was less than 2000 psi....

You can see that with a 3000 psi fill, anything within 6 turns of coil bound, while producing a bit more power, drastically shortens the shot string, and of course wastes a lot of air.... I saw 1000 fps (102 FPE) with everything maxed out at 3000 psi, however.... but the gun ws using nearly 200 psi per shot, and was down 4% in just 3 shots.... The best setting I could find was 7 turns out, starting from 2900 psi and ending at 1900.... It peaked at 929 fps (88 FPE), and averaged 913 fps (85 FPE) over 11 shots.... but the efficiency was only 0.88 FPE/CI.... This is the first gun I have tried where the area of the transfer and barrel ports (0.219"D) were larger than the area of the throat, which is the equvalent of a 0.203" hole.... I'm pretty sure that is why I couldn't get over 1.0 FPE/CI like I did with the 3/16" transfer port in the initial trials yesterday.... This leaves me with two choices, either drill the throat out to 1/4" (it is currently 15/64").... or refit the 3/16" transfer port.... The former should increase the power even further, and may increase the efficiency slightly.... The latter will drop the power a bit, but increase the shot count and efficiency.... I will wait until I get the new longer barrel to make the final decision.... but since I'm already pushing these pellets into the mid 900s more power may not be the answer.... so I'm leaning towards going back to the 3/16" transfer port with the 25" barrel....

Bob

rsterne · Moderator

This gun will have a shroud, but not one that will affect the noise level of the gun, they are prohibited in Canada, unfortunately.... I will be using a 7/8" OD x 3/4" ID Carbon Fibre tube located on two 3/4" bosses, one on the front of the breech, and the other on the back of this modified Hatsan air stripper.... These strippers are available in .177, .22 and .25 cal (shown), having different inserts for the different bores.... The brass insert is adjustable fore-and-aft to adjust the air gap between it and the muzzle, which can have a dramatic effect on accuracy.... I will be making a .30 cal insert for the stripper on this gun....

The rear portion of the stripper was tapered, but I turned a parallel section with a 3/4" diameter up to where the stripper was 7/8" OD (where the shoulder is).... It will thread onto the end of the barrel on standard 1/2"-20 NF threads.... The shroud will mount between the bosses on the breech and stripper, and when the stripper is tightened up, the shouders will put the shroud in compression and the barrel under tension.... which should stiffen the barrel and raise it's resonant frequency.... In theory, this should reduce or eliminate the effect of barrel harmonics.... which can be quite a problem on long, thin barrels.... In addition, the woven finish of the CF tube should look pretty KEWL.... Once in place, all three tubes will be the same diameter, and also the same width as the breech....

Bob

rsterne · Moderator

Sean and I discussed what direction we wanted to go with this project, and decided that we may as well max out the performance and then he will supply a smaller transfer port with the kit so that people can detune the gun easily for more shots.... We also decided to set the gun up for a 3000 psi fill by using 10-32 screws to mount the valve.... With that in mind, I pulled the valve, installed the larger screws, drilled the throat out to 1/4", and enlarged the exhaust port to 7/32", the same size as the transfer and barrel ports.... Here are the results, still using the 21" PB barrel and the .308" upsized pellets.... We expect a bit more with the 25" barrel....

The power at 3000 psi with the preload at coil bind is now 105.7 FPE.... At 2 turns out, the first 3 shots are nearly constant velocity at 101 FPE, and at 4 turns out there is a very good 8 shot string averaging 949 fps (92 FPE) starting from 3000 psi and dropping to 2100, with an efficiency of 0.78 FPE/CI.... Since the magazine will hold 7 shots, this tune will give a full clip at maximum power, a good choice for hunting.... At 6 turns out, the gun is the most efficient, delivering 10 shots averaging 915 fps (86 FPE) at 1.05 FPE/CI, from a 2700 psi fill and refilling at 1900 psi.... At 8 turns out, the power is down to 78 FPE, still at 10 shots, with a pressure range from 2400 down to 1600 psi.... I particularly liked the way the gun shot at 5 turns out (the black line) where it gave 9 shots averaging 937 fpe (90 FPE) from 2800 psi down to 2000 with an efficiency just under 1.0 FPE/CI....

The barrels are now here, and Sean will be chambering my barrels and I hope to have them within a couple of weeks.... Then I just have to find enough time during our coming busy Tourist Season to do the necessary work to install and test them.... I have two .30 cal barrels coming, one of which Sean will be choking, plus a .25 cal for testing the gun with 53 gr. cast bullets, and the 47 gr. hollowpoint version as well.... I'm really looking forward to seeing the results....

Bob

rsterne · Moderator

I couldn't do any more testing until I get the new barrels.... so I decided to see how this barrel would shoot with cast bullets.... I pulled the barrel off and increased the chamber length with the reamer I made so that it would chamber a 118 gr. Lee FN, cranked the RVA to maximum preload, and fired a string across the Chrony.... It was one of those rare times when having the gun maxed out was the perfect solution, I was rewarded with a beautiful 7 shot string with a 4% ES....

667
680
687
693
687
684
670

The peak energy was 126 FPE, the average was 122 FPE, and the efficiency was 0.80 FPE/CI.... which is not bad considering the power level.... This is almost as much power than I got with my 9mm Disco Double, and more than I got with the .308 version, both of which were 24" barrels.... so I was very pleased with the results.... This really has nothing to do with this project, it was just a matter of satisfying my curiousity....

Bob

rsterne · Moderator

I got my new barrels today from Sean, and all I can say is WOW !!!

He machined the chambers, crowned and choked one of them and taper lapped the bore (1000 strokes), and then even polished the outside.... It looks like Chrome inside and out, I can't wait to see how it shoots.... The JSB .30 cal pellets are a PERFECT fit in the chamber and engage the leade beautifully, leaving good rifling marks on the head, but without smearing the area between....

In addition, he shipped me some of the CF Shrouds, and even cut them to length.... I had previously machined a Hatsan air stripper to fit inside the end of the shroud and made a .30 cal insert for it.... and it slid inside perfectly.... The shroud is 7/8" OD, to match the Disco and Disco Double tubes.... It will end up in compression, with the barrel in tension, when the stripper is snugged into place on the threaded muzzle....

Now to find some time to machine in the O-ring groove in the breech and the barrel port and thread the muzzle for the stripper....

Bob

rsterne · Moderator

We had a slow week at the Motel, so I got a chance to get into the shop and machine the O-ring groove and port into the new barrel today.... I shortened the shroud so that when the air stripper is tightened the muzzle is just clear of the front of the threaded section of the stripper and got to take a look at the new gun assembled for the first time.... It still needs some parts blued, but here is the first photo with the new barrel, shroud and stripper in place.... The gun weighs exactly 8 lbs. and I'm extremely pleased with the appearance.....

The first shot over the Chrony was 1045 fps, but my tank need topping up, so after 3 hours on the ShoeBox, after dinner I got to shoot a set of strings.... I filled to 3000 psi and shot until the velocity dropped 4% below the peak in each case.... The last 4 strings required less than a 3000 psi fill, as I was reducing the hammer spring preload for each subsequent string.... Here are the results....

Maxed out, the first shot was the fastest at 1049 fps (112 FPE).... At 4 turns out, I got 10 shots that were within 4% ES, and they averaged 975 fps (97 FPE), peaking at just over 100 FPE.... The efficiency was 0.91 FPE/CI, filling to 2900 and refilling at 1880.... At 7 turns out, I got 12 shots averaging 912 fps (85 FPE), with an efficiency of 1.11 FPE/CI.... The fill pressure was 2400, refilling at 1520.... I think if I installed a slightly smaller transfer port I could stretch the string to 14 shots (2 clips)....

I'm very pleased with these initial results.... With the first serious testing of the new barrels producing MOA acccuracy out to 100 yards, this should be a serious hunter.... I'm looking forward to getting the new 7 shot magazines from Lloyd at AirGunLab.... He has converted .25 cal MRod magazine to .30 cal, and they look gorgeous.... My breech was made to fit them....

Bob

rsterne · Moderator

Since this gun is going to have a 7 shot magazine, I was curious what I could get for 7 shots starting at 3000 psi (ending at 2250).... Here is the string....

984
1000
1007
1003
997
993
981

Average fps = 995
Average FPE = 101.1
Total FPE = 708
Press. Used = 750
psi / shot = 107
Total CI Used = 786
FPE/CI = 0.90
% ES = 2.6%

I'm thinking this is a heck of a Coyote gun....

Bob

broommaster2000 · Moderator

That thing is epic.

rsterne · Moderator

I finally got back to this project today.... I received some of the converted Marauder magazines from Lloyd at AirGunLab and had a chance to finish them up.... If you are interested in these .30 cal magazines, I suggest you contact Lloyd to enquire about them.... http://www.airgunlab.com/ .... As yet, they are not on the website.... Here is how they look, along with a .25 cal single shot tray I modified by increasing the diameter of the channel to fit the .30 cal pellets....

Lloyd starts with a .25 cal MRod magazine and has a new 7-shot star wheel made by CNC for the larger pellets.... It is engraved with the shot numbers, which I filled in with white paint to make them more visible.... In addition, the clear plastic cover is changed to read .30 cal, and the kidney bean shaped hole is enlarged as well.... On the magazines I received I still had to drill out the hole in the back to feed the larger pellets.... I centered off the original hole, and used a 5/16" end mill for that.... When I reassembled the magazines I added 1/3 turn of tension to the spring to compensate for the larger pellets.... It may have not been necessary for feeding, but they certainly do advance with authourity, and the skirts are so thick on the .30 cal JSBs I can't imagine the slight increase in spring tension causing any damage....

I'm in the process of mounting a scope and hope to do some testing soon....

Bob

renderbrandt · New Member

Bob, this is a fantastic project you're undertaking.
A couple of quick questions, will we be able to buy the upgraded valve pieces to allow for a 3000PSI fill for discos in other calibers than .30.

Being new to this particular forum, does Sean (your partner) have a website where he will be marketing this?

Thanks so much!

I am subscribing to this thread:-)

Regards

T