Which 6mm?

[shootinmiles]

A big thanks to everyone who replied to my question. I have decided against the larger cases for the reason of barrel life. I do not need a round that requires the hassle of fire-forming. I have a buddy that shoots p-dogs with a 6BR, he loves it, super accurate, hits harder than 22 cal bullets.

However, I am not going with the 6BR for two reasons. 1. My smith believes that it is a tad on the small side. 2. I don't want to copy my shooting buddy.

The BRX sounds like a great cartridge, but seems to be more of a hassle to reload that first time. From what I have seen, the 6-250 will have nearly the same ballistics but with the ease of only having to neck it up. My smith says that I will not notice any accuracy differences between the two (I have seen his work perform and he does an amazing job).

In conclusion, I will be going with the 6-250. The only decision I have now is what twist. I can get more velocity out of a 1/13 if I want to just stay small and smack the critters up close (I film all of my shots so I don't like to shoot that far due to not being able to see them in the camera that well); or to go ahead and get an 8 or 9 twist that will allow me to shoot at most everything while losing a little velocity when I go with the smaller bullets. I will most likely go with the fast twist.

I do have a theory: The explosiveness of any given bullet can be increase more dramatically by the RPMs it's spinning when it reaches the target rather than how fast it's moving when it reaches the target. The momentum from the fast movement should carry the fragments in a linear direction consistent with the shot, while the centrifugal force of the spinning bullet should cause the fragments to radiate outwards (explosion) after contact is made.

Now to run some numbers to see what RPMs they are spinning at velocities and twists.

-Miles

PS- I will be posting pics of the gun once it arrives and later of the targets. It looks like the lagging step in the build will be waiting on the McMillan stock. It looks like a 5 month wait...

 

[WyleWD]

From what I have seen, the 6-250 will have nearly the same ballistics but with the ease of only having to neck it up. My smith says that I will not notice any accuracy differences between the two (I have seen his work perform and he does an amazing job).

In conclusion, I will be going with the 6-250. The only decision I have now is what twist. I can get more velocity out of a 1/13 if I want to just stay small and smack the critters up close (I film all of my shots so I don't like to shoot that far due to not being able to see them in the camera that well); or to go ahead and get an 8 or 9 twist that will allow me to shoot at most everything while losing a little velocity when I go with the smaller bullets. I will most likely go with the fast twist.

Good call on the 6-250. Regarding the twist rate.... maybe split it down the middle and go with a 9 or 10 twist. My hunting partner gets some surprising speed and excellent accuracy with his 10 twist 6-250 using R-15 and either 70gr NBT or 75gr Vmax bullets. When he hits a young chuck with it...... carnage.... just carnage is all I can say. ;D WD

 

[CanusLatransSnpr]

A 12 twist wouldn't be bad for you! I run a 12 twist .243 Win. and I stabilize the 87gr. VMax, atleast out to 200 yds. Haven't taken it any further than that. You may not have the velocity with a 6-250 to accomodate that bullet in a 12 twist? Just some food for thought!

Mike

 

[bheadboy]

Mile, there is a formula for the calcuation of twist needed based on the length of the bullet, the dia of the bullet and the major factor is the length, thus, the plastic tips ( the best for splatter) being longer take more twist in a given dia and weight.

for the 70-80's you would be well served to run the formula but to my knowledge the bullet lengths are in a web page- do a search- and i think the 10 twist will give what you need for the velocity and those design of bullets.

i have the formula to calcualte the twist , email me and i will send it to you, i could not get it to attach as it is a work doc.

Bob

 

[lpreddick]

just a note on rpms and twist rate: the formula is RPMS=720xmuzzel velocity/twist. this number is quite large but the kicker is the word "minutes"..rpms. bullet flight time is measured in millisec for short range and mere fractions of a sec for really long range. a bullet actually makes a very few rotations before reaching 400 yds. i'v studied terminal ballistics (the splat factor) for awhile and the amount of varmint dismemberment is related to factors other that rpm. energy=1/2mass times velocity squared. more downrange energy comes from speed. bullet design is next...target vs varmint. shooting varmints with target(higher bc) bullets is risky in that the bullet can go right thru and the varmint has to bleed to death before escaping into his hole...ghogs. varmint bullets are designed to come apart after penetration and thereby "dump" their energy inside...this produces a hydrostatic shock that can destroy the vitals or result in an instant neurological shutdown. i have shot many ghogs that had no external bleeding and finding the entry wound was not easy...not a drop of blood! your chosen cartridge should offer plenty of the above since ther are a lot of 6mm bullets. i like high bc and will hollowpoint target bullets for their flat shooting and terminal performance.

 

[65BR]

Let's see, one REVOLUTION per 8"...1200 feet, so we have 1.5 revolutions x 1200 in 400 yds, so you = 1800 SPINS/TURNS out to 400 yds....yep, just a few turns I'd say.

I can say this on spin, I have run 8 twist 6BRs, spitting 66 and 68gr into 1/4" at 200 yds for 3 shots over and over, AND the same w/the same gun, using 105 Amax's.

The 70 TNT is hell at 3300-3400mv and 8 ROT. Varmints beware

 

[lpreddick]

my point re rpm and terminal ballistics is that bullet spinning is not as effective as bullet speed and it's tendency to come apart is killing the varmint. a bullet from a 1 in 8 twist barrel makes one 360 degree revolution OVER 8 inches before leaving the muzzel. the thickness of a ghogs chest is about 6 in, so the bullet has not quite mad one revolution while in the animal. we know that too many rpms can spin a bullet apart esp j4 jacket varmint bullets...another reason i hollowpoint target bullets for varmints.

 

[shootinmiles]

we know that too many rpms can spin a bullet apart esp j4 jacket varmint bullets..

Which is a main reason why I thought that rpms played a large role in explosiveness. How many rotations a bullet makes inside of a ground hog shouldn't make any difference since most varmint bullets are supposed to provide near instantaneous fragmentation. It's not like a TSX creating a hydrostatic wound channel due to rotations.

I am not a terminal ballistics professional, but I am savvy with physics. I'm still having a hard time understanding how the speed of a bullet in a linear direction can cause the fragments of a bullet to explode outwards. It takes a lot of force to make things change direction. That's why I hypothesized that the centrifugal is what makes the fragments radiate outwards from the center.

I do see how speed can cause the ballistic tip to hit the center of the bullet and cause the fragments to start heading outwards, I just don't see how it that effective. If some bullets can be exploded when shot in too fast of a twist, then I wonder what force is enacted upon that bullet? Logic says that it has to be quite large. I will test my theory on a 1-12 twist 223 and one with a 1-7 twist. Same load, Vmax bullet. Most likely will shoot a water bottle, maybe cantaloupe, and will get on film.

 

[lpreddick]

early on when i was hollowpointing match vlds i did the following: filled one gallon milk jugs with shredded newspaper and wet it down...hopefully near consistency of ghog. shot one with a 6-284 pushing a berger 105 gr vld at about 3400fps, 1in 8 twist. shot another jug with the same except i hollowpointed the bullet to a very thin tip. wish i could figure how to post photos as this was extremely impressive. the jug hit by the match bullet had an exit hole about 1 in dia, while the jug stayed intact. the hollowpoint blew the entire back of the jug apart and there was a cavity inside the wet newspaper the size of a cantalope...VERY IMPRESSIVE!!! the deformation of the bullets tip starts the deflagration and "dumping" of the bullets energy. once the tip is destablized the spinning definitely contributes to this process. spinning apart a match/vld bullet does occur as some long range shooters attest. varmint bullets by their nature are more prone.

 

[Nomad47]

Please excuse a temporary change of subject...... lpreddick, what barrel length was the 6-284 shooting 105s at 3400? And which powder, if you don't mind?

 

[signut]

6BR, no turn neck. Accurate, easy on barrels, no recoil. What's not to like ? A 6Br will shoot to " way over " 600 yards , accurately, and that's all you need. But that's just my .02.....

 

[lpreddick]

Nomad47: kreiger 28 in and rel 22, 51.5 gr about 15 thous into lands. the 6-284's barrel life is short if shot in competition, but like your 6xc i only shoot it at long range ghogs...522yds and 532yds being the longest. your longest shots far surpass these. wish i could buy you motorhome but i'm having a new 22 BR 14 twist made and it will require my attention.

 

[Nomad47]

Thanks, lp. At 51.6 gr of RL-22 I was running 3316 MV out of a 26" Shilen, .020 off the lands with 105 Hybrids. Best RL-22 load seems to be around 50.2 grains at 3220 MV. I just started trying H4831sc and I think my barrel might like it a little better than RL-22.

 

[PALarson]

There is a lot of very good information on this post, all has it's place. I have had 6-284 good long accuracy, very poor barrel life, I have 6BR, great 600 yard competition gun, a little weak for 1,000. I have made several 6BRX's for my self and customers, a real pain to blow out cases without any problems. I have and use regularly 6 XC appears to me to be as accurate as the 6 BR and BRX but a couple of hundred feet faster to buck the wind better. Cases are easy to blow out no problem and new cases are ready to load and shoot from Superior systems. My vote is for the 6XC.
Paul Larson Mid range and Long Range HM
'

 

[Nomad47]

Definitely no arguments from me concerning the 6XC.