First big bore build

[mackeral5]

That is a great looking gun.  it is amazing that you designed and fabricated most every part,  using tech ranging from old school casting to 3d printing.  Again, amazing work.

[Scotchmo]

I just set up the gun with a bottle regulated to 1200psi. It shoot the 34gr jsb at about 1040fps. I'll be testing that velocity on Monday using my new Labradar.

I also got my Arsenal mold in and did a first run of about 170 slugs. Still getting used to the mold and had some trouble fully filling all five cavities with the small ladle (unless pot is very full). Full cavities come out at about 71gr with the unknown lead that I had on hand. About .256 diameter, unsized. At 1200psi, they are at 770fps. They will be shot at 2200psi when I'm done testing pellets at 1200psi.

[rsterne]

Slugs look pretty nice.... good job!!!!

Bob

[Scotchmo]

Slugs look pretty nice.... good job!!!!

Bob

I've been shooting the 70gr slugs recently and they are starting to look promising. I cut the chamber to fit them better. I'm lubing them with my bee's wax/white grease mixture that I use in my pistol reloads. I'm sizing these Arsenal slugs down to .254. I weight sorted a batch and used only those between 70gr and 71gr for this session.

I'm still messing with the regulator on this gun. Current settings have it at about 2000psi (935fps, which is slower than I really wanted to test), but it creeps up to 2200psi (975fpe) if it sits for any length of time. So the first two or three shots tend to be higher velocity. You can see that in the Labradar readout of the first 25 shots from the last shooting session. The BC columns were entered manually, so it's possible that there are errors there.The Labradar tracks these slugs well out to 50yds and still OK at 75yds, but it drops some of the 100yd readings. The 958fps and 977fps were after the rifle had sat for awhile and the regulator had crept up. Outside of that, it was pretty consistent. It is surprisingly efficient at this setting. I should get over 100 shots at 135fpe on a complete fill.

The first large target was 10 shots at 100 yards in approx 0-3mph winds.

The second picture is of 4 different 5 shot groups at 100yds. I adjusted the zero between groups, but still not quite on. Averaging under 1moa group size at 100yds is good. I just need to get it centered on the bullseyes.

Fill pressure: 3800psi
Regulator pressure: 2000psi
Projectile: 70.5gr 420257 BT
Velocity: 935fps
Energy: 135fpe

Next time I plan on comparing these slugs to some 82gr Griffin BT slugs at the same settings.

[Scotchmo]

I did some more testing today. Out to 200m, at sea level. Wind was 8mph.

The 82gr Griffin slugs were tested but they did not do well at this lower power level. The 70gr 257420 slugs continue to shoot well at this power level (920fps). The regulator is settling in at a lower pressure than I wanted so it's going to be re-shimmed before the next outing.

Targets were shot at 50yds, 100m, and 200m. I miscounted the number of shots. I think the 5 shots at 100m might actually be 6 shots.

[BigBird]

Nice groupings however you missed the quarter on every one 

[Scotchmo]

Nice groupings however you missed the quarter on every one 

Were the quarters located on the bullseye, at 50yds, I would have hit the quarter on 100% of the shots. At 100m, about 50% of the shots. At 200m, 1/10 of the shots.

I analyzed the shooting to see where I can tighten up the groups. The wind was up to 8 mph that day. The horizontal dispersion of the 10 shot group at 200m was 5". That could be accounted for by a 4mph wind variation. The vertical dispersion was 2.5". That could be accounted for by an 11 or 12fps velocity spread. I had about a 10fps velocity spread for those 10 shots. Wind is beyond my control, though I could use some flags and attempt to compensate. A higher BC projectile would help, but I'm sticking with these for the time being. The vertical spread can be improved by cutting down on the fps spread. I used slugs that were sorted to 70.0-71.0gr for that test. Next time I'll use slugs sorted at 71.0-71.25gr. I also worked on the reg some and it now appears more consistent at 2200psi. Goal is to get consistently under 1moa at 100yds. I think I can do that.

If I can keep my velocity spread under +/- 8fps, and winds are under 2mph, I think 1moa at 200m is also doable with this rifle.

[mackeral5]

Your build is really coming together.  Every time I see you post I remind myself that you even CAST your own breech!!!!! 

My .257 Bulldog is an ongoing saga of frustration.  It is a dual caliber gun with .257 probe/barrel/magazine and .357 probe/barrel/magazine.  I cannot get the .257 setup to outshoot the .357.  Recently I shot a .800" 5 shot group at 100 yards with the .357 barrel.  In .257 the smallest is around 1.25-1.5".........I'll beat it eventually...I hope.

[Scotchmo]

Your build is really coming together.  Every time I see you post I remind myself that you even CAST your own breech!!!!! 

My .257 Bulldog is an ongoing saga of frustration.  It is a dual caliber gun with .257 probe/barrel/magazine and .357 probe/barrel/magazine.  I cannot get the .257 setup to outshoot the .357.  Recently I shot a .800" 5 shot group at 100 yards with the .357 barrel.  In .257 the smallest is around 1.25-1.5".........I'll beat it eventually...I hope.

Last week at 2000ft elevation, in calm winds, I did some 5 shot groups at 100yds and did four groups. They were all decent.

0.520 c-to-c
0.741 c-to-c
0.851 c-to-c
1.134 c-to-c

That looked very promising. The most recent groups at 100m and 200m, at sea level, with more wind, were also promising. At 50 yards, this Armada HP now looks to be even more accurate than my FT rifle.

I think I made some substantial accuracy progress with the latest attention to the lead-in and slug sizing. If you've got the TJ 25-20 1:14 barrel, you can do it too. I quit trying the multi role approach (pellets and slugs) and just went for slugs for now. I cut the breech using an unmodified #5 taper pin reamer. I did it in steps so as not to cut too much. And tested between each steps with different depth sizing of the slug. I had made a tapered sizing die using the same reamer. I roll the pellet in bee's wax/lithium grease mixture. I use an adjustable push mandrel to push the slug into the sizer the appropriate depth. I use another mandrel to tap it back out of the sizer. The sized 257420 slug goes into the lead-in with about 1/4" of push needed to seat it. It's a smooth operation with the lube. The final OD of the slug after sizing is .2535" at the rear band. I started with that diameter as it gives about the same cross sectional area as the barrel/grooves. So minimal deformation in the slug and still a decent seal.

[BigBird]

Nice groupings however you missed the quarter on every one 

Were the quarters located on the bullseye, at 50yds, I would have hit the quarter on 100% of the shots. At 100m, about 50% of the shots. At 200m, 1/10 of the shots.

I don't doubt that.  Impressive build and accuracy out of this!

[Scotchmo]

I'm narrowing in on a preferred tune with the 257420 slugs. I've pushed it out to 200m (218 yards) to get more data. Today was a little windier (3mph-11mph measured) than I wanted but even that provides usable info. I used a 100yd zero and and held over starting with 24moa for three sighter shots. I decided to start with a wind hold-off of about 2moa. I varied the windage by about 1moa through the test and and elevation by about 1/2moa. I shot 5 targets of record with 5 shots each. I got lucky on the last target as I finally got under 1moa at 200m.

If the gun sits for more than a few minutes, I always fired air only for one shot in order to reset the reg after it creeps. It holds close to 2300psi for a couple minutes but will creep up to 2400psi overnight. The Cothran powerhouse valve requires a very consistent pressure in order to maintain a consistent velocity. I shot lubed slugs sorted to the 71.0-71.5gr range. I'm getting a standard deviation of only about 3fps in velocity. I can probably tighten that up a little more by sorting to within +/- 0.1gr. The Labradar tracks these slugs well out to 50yds, most out to 75yds, and some out to 100yds. BC is mostly in the 0.160-0.170 range.

Hoping for a calmer day next time. Plus, I plan on clicking to zero at 200m, and more precise slug sorting. Velocity is currently a little higher than I think I need for 200m, but the long term plan is to get out to 300m. I hope to attend the Rocky Mountain Airgun Challenge, and the EBR this year. With plans to enter the big bore competition, and they go out to 300 yards, so I want to be prepared.

[Scotchmo]

Closer examination showed that "flyer" was a tear in the cardboard and not a shot. The extra shot was taken at target #3 by accident. So #3 has 6 shots and #4 only has 4 shots.

[Scotchmo]

Since my last testing day, I had bumped up the regulator pressure to 2400psi. I had some of the lighter reject 257420 slugs that I was temped to remelt. The non-rejects are in the 71-72gr range. These rejects were mostly in the 69.5-70.5 range. I had some time, so I decided to go to the range and shoot them instead of recasting. It ended up being 100 degrees F at the range, and 10-15mph winds. Accuracy was not bad considering. The reg stays between 2350-2400psi for most of the shooting. It creeps up to 2500psi if it sits long enough. After a few shots, it stabilized at about 993fps.

PSI before first shot: 3600
PSI after last shot: 3000
Gun tank volume in cc: 2000
Pellet weight: 70
Total number of shots: 52
Average FPS for all the shots: 993
avg FPE/cuin per shot: 1.58
avg bar-cc/FPE per shot: 10.4
avg FPE per shot: 153.3
avg PSI used per shot: 12

This current setting gives great efficiency for an HP airgun. Though I rarely fill past 4000psi, a full fill of 435psi (300bar), shooting down to 2400psi should give me over 150 shots at over 150fpe.

[BigBird]

Wow!  Thats some nice efficiency!  You could do some prairie dog shooting with that.

[Scotchmo]

Wow!  Thats some nice efficiency!  You could do some prairie dog shooting with that.

Probably too good to be true. The high temps at the range had something to do with the apparent extra high efficiency. I just checked the tank and it is closer to 2900psi after temperatures cooled down today.

This would be closer:

PSI before first shot: 3600
PSI after last shot: 2900
Gun tank volume in cc: 2000
Pellet weight: 70
Total number of shots: 52
Average FPS for all the shots: 993
avg FPE/cuin per shot: 1.32
avg bar-cc/FPE per shot: 12.1
avg FPE per shot: 153.3
avg PSI used per shot: 13

Still good at 1.32 vs the too good 1.58.

[rsterne]

Makes more sense.... Still excellent numbers....

Bob

[Scotchmo]

The first competition test of my big bore build went great. I entered four events at the Rocky Mountain Airgun Challenge (RMAC) last week. I brought a modified Hatsan Bullboss to shoot three different pellet matches and had my custom Armada .257 for the Magnum Slug Challenge.  After dedicating the Armada to slugs only, I sorta rushed the build of the Bullboss pellet shooter. Shooting 18.1gr pellets was fun, but not ready for prime time at the match. The days of competition were long and after struggling with the speed shooting match, I decided to drop my spot in the next morning's precision pellet match in order to rest up and concentrate on the Magnum Slug match.

The match was divided into two heats. Each heat had four stages with two targets in each stage.  No sighter shots allowed. We shot each target twice for points so every shot had to count. Targets were arranged somewhat radially down range. Target distances were approximately 75, 100, 130, 187, 200, 229, 267, and 290yds for the eight targets. I said “approximately” since each shooter had their own bench for the duration of the match. So where you sat in relation to the radial spread would determine the actual target distance.

I had generated dope at my local range in California. Sea level, 70 F. The RMAC range was at 4600ft elevation, 85F, with some targets up the hillside at angles up to 15 degrees. My Iphone was wonky after dropping it the prior week so I did not want to rely on it for calculating the trajectory changes and compensation during the match. I translated my D.O.P.E. according to the local conditions the night before and made notes on paper for various distances and angles. At the match, I used my laser range finder to measure the range and inclines, and then picked correct click values from my notes, before engaging each target. I think two of my misses were from trajectory data errors as it seemed to be hitting consistently high by up to 2moa on the mid range targets (187yds-229yds).

I ended up missing only three out of the sixteen target shots. Good enough for 2nd place and $1000 prize!

https://www.rockymountainairgunchallenge.com/rmac-2021

[rsterne]

WELL DONE SCOTT !!!!!!!!!!

I see a few familiar names that placed below you.... 

Bob

[Rob M]

well done !!!!!!

[Mada]

Great job Scott!!