First big bore build

[Rob M]

awesome Scott, yes Don cothran designed the valve way back when.. He was the first to market with any type of balanced valve.

[BigBird]

Great info in that video.  That answers a lot of questions too.  That is one great looking gun!  II love your pill container.

It would be fun to take that thing out coyote hunting or even hog hunting over a hog wallow!

[Scotchmo]

I was running near the minimum FPE required for these big bore matches. For the RMAC, I shot 71.5gr slugs at 980fps (about 150fpe) . For EBR, I was at 1010fps (about 160fpe). There was one target I hit, but the target moved an insufficient amount to get the points. A couple other targets hits were marginal. I want to try to eek out a little more momentum, and retain the accuracy. I ordered an Arsenal 257388 five cavity mold last week and it got here already. I raised the regulator pressure from 2450psi to 2900psi, and bumped up the hammer voltage from 60v to 63v. Pressure can safely go to 3000psi, while the hammer circuit is now maxed out at 63v. I did a couple of test shots in my garage over the chronograph. The new 86.5gr slugs are at about 995fps (about 190fpe). I hope to take it out to the range on Wednesday and do some more extensive testing.

[Gone Fishing]

I was running near the minimum FPE required for these big bore matches. For the RMAC, I shot 71.5gr slugs at 980fps (about 150fpe) . For EBR, I was at 1010fps (about 160fpe). There was one target I hit, but the target moved an insufficient amount to get the points. A couple other targets hits were marginal. I want to try to eek out a little more momentum, and retain the accuracy. I ordered an Arsenal 257388 five cavity mold last week and it got here already. I raised the regulator pressure from 2450psi to 2900psi, and bumped up the hammer voltage from 60v to 63v. Pressure can safely go to 3000psi, while the hammer circuit is now maxed out at 63v. I did a couple of test shots in my garage over the chronograph. The new 86.5gr slugs are at about 995fps (about 190fpe). I hope to take it out to the range on Wednesday and do some more extensive testing.

Nice

[Scotchmo]

Before I go on: I stuck with the 71.5gr slugs as I couldn't shoot the 86.5gr quite as accurately. They required more energy, and never had the docile, easy shooting quality of the 257420 BT.

Now the real story:

Another great outing for the .257 at the 2022 RMAC match. I teamed up with two other shooters from “Team Centercut” as a way to increase our chances of a win. There were three relays for the Magnum Slug Challenge. We had one shooter in each relay, all three shooting my custom .257 Armada. This was the biggest slug challenge yet with 52 competitors, including factory sponsored entries.

It was serendipity that Dave Wilson ended up shooting it for the win. I'd originally planned on teaming with just Mike for entries in two relays for this year's slug match. With three available relays, I asked Dave Wilson at the match if he wanted to do the same. For Dave's relay, he would shoot and I would spot. Dave was able to get in as a late entry when another competitor dropped out.

I knew if there were no mistakes made, and with some luck on our side, we had a chance of getting in a perfect score for the first time at an RMAC Slug Challenge. Mike had it the toughest as the winds were worse early on. Conditions were a little better for me. I thought we both shot well but neither one of us were clean. Conditions improved a little more by the time Dave's relay was up. Dave seemed nervous for his first big bore match but he was rock solid when he got on target. No mistakes and he broke every shot perfectly. I was both proud of my gun and proud of Dave. While I was happy with my third place performance (thank you Mike for spotting for me), Mike and I were both thrilled with Dave's win.

A couple of days before the Slug Challenge, we got some teasing (all in fun) for shooting a “babybore” as some other competitors held their .457 slugs next to our little .257 slugs. Mike retorted “I don't know WHO is going to win, but I know which GUN will win.” I too had confidence in the gun so I knew we had a good chance, but also knew it would take some luck as well.

For our teams (don't know about the other teams), shooters and spotters split the purse. So it covered most of my out of pocket for this adventure. It was a great finish to an almost two week trek that started with a drive from my home in Los Osos to shoot the Nevada State FT match at Battle Mountain. That's a story in itself. Then it was on to Utah. The performance of our team at RMAC made the solo drive home from Utah to be not so bad as I reflected on the previous two weeks.

[Loren]

Awesome!!  Thanks for sharing the story.

[Brian W Cook]

Congratulations on the RMAC check .   I gave up on trying to get my .257 talon to shoot reliably the week before the shoot  and ended up throwing my old .458 together that i used to use in the past Lasso shoots .   It ended up with a slightly too big poppet and was getting inconsistent velocities.   Had a great time and got to meet some great shooters . 

[MJP]

Awsome outcome on a nice gun build!

Marko

[rsterne]

Great job, Scott.... Way to go!!!!

Bob

[Scotchmo]

So, a new dilemma. The 257 “babybore” has been beating up on the bigger bore airguns at the RMAC and EBR long range slug matches. I plan on shooting in the upcoming EBR in October. In the past, there was a 150fpe requirement for both RMAC and EBR. EBR just changed their rules to say “200 ft-lbs minimum is required”. Even the 150fpe was never checked in the past. Now they say it's a requirement and they will check any rifles suspected of being <200fpe. They gave a bunch of reasons for this new rule change, but I think the real reason is that they don't like having their “bigbore” 357 and 457 rifles spanked by the “babybore” 257.

So now it's – GAME ON.

I built this gun for that specific purpose. It is based on the knowledge gained over the years by the likes of John Bowman, Doug Noble, Bob Sterne, et al. I borrowed heavily from what worked for them and it paid off.

I've got two months to get up and running over 200fpe (I'm going for 220fpe+). I'm going to look at a few options.

My favorite 71gr 257410 BT won't be a good choice for >200fpe in an airgun. I do have the 257388 at 87gr that has a higher BC and shoots almost as accurate as the 257420. I'd need to push it at >1020fps which is going to be a stretch for a 3000psi rated Marauder air tube even with a 36” barrel. I got about 190fpe in earlier tests at 3000psi. I could push the pressure to 3200psi and that should be just enough. I'll need more hammer energy to reliably open the Cothran valve at 3200psi. More capacitance at least and preferably higher voltage, so some rework of the electronics. My preliminary calcs show that it might get me there but it's marginal. There is also the Arsenal 92-257 “Noble” slug, but that long of a slug would be marginal in the current 1:14 twist barrel.

I'll likely need a bigger caliber and/or longer barrel. A 48” .257 barrel and the 257388 would get me there with room to spare. Now if I'm going to go with a new barrel there are other options. The current gun design limits me to about .308 max caliber and 0.750” max projectile length. I'll go with any length barrel that TJ or some other manufacturer can supply. Best if I keep the 225cc plenum pressure at 3000psi or lower.

Options that I'm considering:

257388 87gr in a 48” long 1:14 TJ barrel?

257-92gr “Noble” slug in a 1:10? twist barrel, 36”+ long?

284-96gr “Bowman” slug in a 7mm 1:14 twist TJ liner, 36”+ long?

308-135-FN-BF1 (not currently available) decked to 0.75” long, in what barrel?

None of the options have the available/proven history/data of the 257420 in the 1:14 TJ barrel. I want to hear any info/data/experience with the above options.

I'm liking the idea of a “7mm AirMagnum”. I'm looking for some GTA forum collaboration on this.

[bear air]

GETEM!!!! Congrats on your rifle coming together and on all the success at RMAC. You built this gun Scott, I'm sure you can get the power level up and continue to kick butt.

[MJP]

What about the 257 98gr Ideal spizer that Doug and Cedric shoot? I have gotten that stable out of 14" twist barrel pushin 980fps ish.

Marko

[Scotchmo]

What about the 257 98gr Ideal spizer that Doug and Cedric shoot? I have gotten that stable out of 14" twist barrel pushin 980fps ish.

Marko

I open to all options, but #1 - The current receiver is setup for 0.750" long maximum projectiles. #2 - Even if it were stable enough (at this time, I prefer 1.7-2.0), I don't think I can safely run enough pressure to reach that velocity without some other changes.

[rsterne]

I believe Doug Noble now uses a 13" twist for the 92 gr. "Noble" slug instead of the 14", but you might ask him about shooting it in a 14".... He might also know about the 96 gr. "Bowman" 7mm, which is a scaled up 257420, and a known good performer.... TJ's have a 7mm with a 14" twist that will match it well.... If you want to stay with .257 cal, and a 10-12" twist, NOE have the very good 260-105 moulds in stock.... https://noebulletmolds.com/site/product-category/bullet-moulds/257/260-105-fn-d2/

I hate the way EBR make such arbitrary changes when the guns they want to win get their butt kicked....  .... Given an equal playing field, I would be going 6mm or even .224 cal to get the sleeker Form Factors for the same SD, along with less recoil and air use.... 

Bob

[Scotchmo]

I believe Doug Noble now uses a 13" twist for the 92 gr. "Noble" slug instead of the 14", but you might ask him about shooting it in a 14".... He might also know about the 96 gr. "Bowman" 7mm, which is a scaled up 257420, and a known good performer.... TJ's have a 7mm with a 14" twist that will match it well.... If you want to stay with .257 cal, and a 10-12" twist, NOE have the very good 260-105 moulds in stock.... https://noebulletmolds.com/site/product-category/bullet-moulds/257/260-105-fn-d2/

I hate the way EBR make such arbitrary changes when the guns they want to win get their butt kicked....  .... Given an equal playing field, I would be going 6mm or even .224 cal to get the sleeker Form Factors for the same SD, along with less recoil and air use.... 

Bob

If you have it, can you message me Doug Noble's contact info?

The 260-105 is too long to keep the change over simple. I'd like to stick with 0.75" maximum projectile length.

Tough enough to get over 200fpe in .257. A heavy 6mm or .224 would be hard to push fast enough. A little bigger caliber makes it easier to get the 200fpe+ that I need. Even the 0.16 BC from the 257420 has proven good enough. It would be nice to keep the efficiency and enough FPE. If I go 1:14 TJ 7mm, I'll probably start with the 96gr Bowman slug. I'd like a slightly lengthened Bowman slug in the 100gr-105gr range. Specs: 0.284 caliber, 0.740" long, tangent ogive radius, 2 to 3 diameters. A 284-105-FN BBT or there about. G1 BC in the 0.25 range.

[Scotchmo]

TJ said they could do a 48" 7mm liner. I told them to make it an order and invoice me. I also ordered a 7/8" diameter, 48" long DOM tube for the barrel, and the Arsenal 96-284 "Bowman" mold. My other Arsenal slugs seem to cast about 2% over mass with the lead that I use so probably about 98gr when cast.

With the full 48" barrel, I'm expecting between about: 970fps (about 205fpe@2550psi), and 1070fps (about 250fpe@3000psi). With some effort and luck, I should be able to get things up and running with some time to test and dope for the 2022 EBR.

There were 4 standing/offhand shots and 4 standing with shooting sticks at the last EBR match.  With the 36" .257 barrel, the rifle weighed about 23.4 lbs. aired up, and that was a hand full for those standing shots. I only missed 2 of those 8 shots, so maybe got lucky. With the 48" .284 barrel, it will weigh about 25.1 lbs. I contemplated going with a 3/4" diameter barrel, rather than 7/8", and that would save about 2lbs. But it's hard to beat a heavy/stiff barrel for accuracy off the bench, and that is where this rifle will spend most of it's time. I might even need to add a couple more pounds in the buttstock in order to balance it out for the offhand shots. Would still be lighter than a 50 cal Barret rifle.

[bear air]

Given the competition is a little under a year away, you could do some weight training to strengthen those muscles needed to shoot it off hand.

[Scotchmo]

Given the competition is a little under a year away, you could do some weight training to strengthen those muscles needed to shoot it off hand.

Actually, the 2022 EBR is in two months, and that is the match where the minimum was raised from 150 ft-lb to 200 ft-lb. Not much time to workout in preparation for that. The 2022 RMAC match from last month listed 140 ft-lb as the minimum, but if the 7mm (.284) conversion works out, I'll likely shoot it at that match next year as well.

[bear air]

Oh, I misunderstood it was for a different competition coming up soon. Oh boy... Youve already proven to be a very spirited and capable fellow Scott. I'm sure you got this too.

[rsterne]

Scott, I sent the drawing for the 7mm version of the bullet we discussed to Al at NOE to ask if he could do a special run to get it ready in time.... I copied you on the email.... It doesn't hurt to ask!.... 

Bob