Author Topic: Improved BBT ? (Read 3248 times))

rsterne · « **Reply #20 on:** April 05, 2021, 08:49:07 PM »

The last couple of designs are highly specialized, small caliber, long range slugs.... Here is the 3.5 caliber long FN design, with a shortened HP version shown in red....

As before, I have shown the weights for other calibers in blue.... I have one more, even longer version, that I will post soon....

Bob

mackeral5 · « **Reply #21 on:** April 05, 2021, 10:19:47 PM »

Quote from: rsterne on April 05, 2021, 07:05:13 PM

If NOE ever get caught up and are looking for new designs, then it would be possible.... Otherwise you would have to have it made by Arsenal, and then send it to Erik at HP Mold Services to have the HP system installed.... If the radiused corners at the Meplat were reduced, then Accurate would be able to make it as well (they have a 0.180" minimum diameter for the Meplat).... but again, no in-house hollowpoints.... I can supply you a dimensioned drawing for the FN version of the 2.0 cal. long in .357, for either company, if you want one....

Bob

Bob, I would like to try a 2 caliber new version BBT in .357. I'll add it to my budget, lol... should be able to place an order with Arsenal in a couple of weeks which in theory would put a mold in my hands by mid summer. I will be able to compare with your original 174gr BBT. Thank you for your help.

Mike

rsterne · « **Reply #22 on:** April 05, 2021, 10:39:28 PM »

Mike, PM me your email and I'll send you a dimensioned drawing for a 2.0 cal long BBT in .357 caliber....

Here is the last of my new designs, a 4.0 caliber long version.... Once again, if you want a larger HP, the design can be shortened to provide a 50% Meplat, as shown in red....

The weights of other calibers are shown in blue, and the twist rates would be proportional to the caliber....

Bob

Greg_E · « **Reply #23 on:** April 06, 2021, 01:18:16 AM »

The 1.5 version looks pretty close to the Lyman 356637 https://castboolits.gunloads.com/showthread.php?353935-Lyman-356637-147gr-9mm-design&p=4282660&viewfull=1#post4282660

I have this mould but have not had the chance to cast anything yet. They also use this for the Lyman 9mm Devastator which is a very difficult mould to find. I may need to have mine machined for pins and get flat and hollow point pins made.

https://www.natchezss.com/lyman-devastator-hollow-point-pistol-bullet-mould-9mm.html

rkr · « **Reply #24 on:** April 06, 2021, 05:29:37 AM »

That is interesting about the rebated boattail. Now that 2.5 caliber bullet is quite close to 257420BT version and I got surprisingly good BC values (0.17-0.18) when we tested them with labradar. After reading this thread I came to think that my bullets were cast with .260 size mold and then sized down to .258, thus making the rebate 0.02 smaller. Could that perhaps be the reason as it would make the tail pretty close to what you have drawn here? Also, it seems that 2.25 caliber length bullet of this design would be a reborn version of 257420.

YEMX · « **Reply #25 on:** April 06, 2021, 09:25:35 AM »

I've emailed Arsenal with regard to the 1.5 caliber long .243 slug design. Just waiting to hear back. They would be able to fit in a standard magazine with no other mods, other than to the center wheel- but one could be printed... I'm really excited! I've also got a 6mm test mule that'll be finished soon! My dream of testing a mag-fed 6mm will come to fruition soon!!

rsterne · « **Reply #26 on:** April 06, 2021, 05:15:10 PM »

Yes, the 257420 is in between the weights of my new 2.0 cal and 2.5 cal long BBTs.... The BT versions I drew for Arsenal and NOE are a bit lighter than the original Lyman.... All the 257420 versions are great slugs, IMO, the "go-to" rounds for .257 airguns.... I started with a 0.015" rebate step on my BBT designs, and reduced that on the 257420 BTs.... now even further with the new BBTs, and less taper to the boattail, to hopefully keep it within the 8 degrees recommend by Miles....

Bob

sb327 · « **Reply #27 on:** April 06, 2021, 06:26:06 PM »

I’ve got a cutter and a couple of sets of blocks ready for when I get back home. It won’t be as close to dimension as a cnc but hopefully I didn’t butcher it too bad.

I scribed the dykem at the bullet base in first pic. Second pic it’s cut, relieved, honed and ready to go.

Thanks again Bob.

Dave

rsterne · « **Reply #28 on:** April 06, 2021, 06:45:33 PM »

So you are using what amounts to a profiled "D" drill (except cutting on the side)?.... Turn the bullet shape and then machine off half of it?.... It looks like you removed the "trailing" edge as well?.... Simple method, as long as it is effective and cuts a good cavity....

What steel are you using, and how do you heat treat it?.... Tempering temperature?....

Bob

sb327 · « **Reply #29 on:** April 06, 2021, 07:15:21 PM »

It’s just O-1 drill rod. Having made lots of my own reamers/cutters over the years, I just ‘eyeball’ critical (non mag) on something this small with a torch. Light oil at room temp quench. 400 degrees for an hour or so.

Cutting a mold with one takes time. I predrill as far as I can just under the finished base diameter (or last lube groove for fb bullets). Setting cutter in toolpost is critical. It always takes me a while to get it exactly like I want and locked down. Then slowly plunge to depth, set a stop and start letting it cut. About .0005” at a time letting it clean up each time. Tedious and very easy to break a cutter. (That’s from experience, lol)

This is the only way I’ve made molds, there are for sure better ways but I have had good results from this process.

Dave

sb327 · « **Reply #30 on:** April 06, 2021, 07:21:07 PM »

Oh, and yes, the back and bottom are relieved extensively to fit through the small base hole.

Found a pic of a bbt I made for my .22

Dave

Greg_E · « **Reply #31 on:** April 06, 2021, 11:19:31 PM »

I would like to see the machining process of making the form tool and cutting the blocks.

rsterne · « **Reply #32 on:** April 07, 2021, 12:44:53 AM »

Me too!!!!

Bob

rkr · « **Reply #33 on:** April 07, 2021, 02:53:38 AM »

Quote from: sb327 on April 06, 2021, 07:21:07 PM

Oh, and yes, the back and bottom are relieved extensively to fit through the small base hole.

Found a pic of a bbt I made for my .22

Dave

I've been wondering, what if you were to make those mold block sides of two parts? First drill the front part of the bullet and then the rear part and bolt them together, would that work? With that methodology and a design with no bands (like in RB swaged bullets or in case of this design with a bore riding front of the bullet) you could just drill those bullet fronts and rears and then bolt them block sides together for a complete mold, no need for a milling setup or worries about breaking the "drill".

rkr · « **Reply #34 on:** April 07, 2021, 04:59:52 AM »

Quote from: Greg_E on April 06, 2021, 11:19:31 PM

I would like to see the machining process of making the form tool and cutting the blocks.

Here's a set of videos:

sb327 · « **Reply #35 on:** April 07, 2021, 08:30:42 AM »

I essentially use the same process as the fella in that video. Biggest difference is that I use a lathe for boring with a four jaw.

There are plenty of videos/tutorials on making a d bit reamer, but for this purpose, it needs extensive relief on the bottom and sides. I only leave about .020-.030” cutting profile. The bullet shape dictates how much relief is needed.

For the block, I just start with a long section of 6061. Cut the handle slot full length. Flip it and face it off square. I crank fast to leave a rough finish. This helps to vent went pouring. I drawfile but leave the swirling.

I then cut into sections with bandsaw, square the ends and install two pins, opposite corners. I drill/ream to .2490” and press .250” drill rod. Use shorter length than block, then press/hammer in to just enough protrusion for snug fit. Oh yeah, chamfer all holes where they meet inside.

At this point I will form the cavity. If all is well there, I will make a sprue plate. I just make a plate (no hole), install and get it fitted. Then align cavity with spindle, close plate, spot drill and then proceed to make the sprue hole/funnel.

I probably missed quite a bit there so if there are specific questions, I’ll try to answer.

Dave

Here is a mold block showing pin protrusion and the cutter being profiled.

JuryRigger · « **Reply #36 on:** April 07, 2021, 10:46:52 AM »

Way too cool; following along

Thanks for sharing Dave!!
Jesse

rsterne · « **Reply #37 on:** April 07, 2021, 02:27:53 PM »

Super impressed....

.... Beyond my abilities, for sure!!!!....

Bob

rsterne · « **Reply #38 on:** April 12, 2021, 12:54:03 PM »

I had a request for a BBT even shorter and lighter than the 1.5 caliber long version on the previous page.... When I looked at the requirements (a .30 cal that was only 0.375" long), I quickly realized that the only practical way would be to simply remove the boattail and go with a flat base.... Here is the result....

The length would be almost exactly as requested in .30 cal, and it would weigh 59 gr. as a HP and 63 gr. as a FN.... Trying to incorporate a boattail in such a short slug would not leave enough bearing area to keep the slug straight in the bore, IMO.... Something in between in length (~ 1.4 cal.) would be able to use a short gas-check style stub and stay within the 8 degree convergening cone recommended by Miles....

Bob

Greg_E · « **Reply #39 on:** April 12, 2021, 12:58:57 PM »

This last one is pretty much the same as the GT bullets in 357, MP has a design but if course out of stock.

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Improved BBT ?

Author Topic: Improved BBT ? (Read 3248 times))