Why 6um/.25-7prc?

[eoperator]

I was looking to order up either a 6um or .25-7prc 20"barrel but when I start crunching some numbers I have to ask if there is an advantage over 6.5prc? Am I missing something, Are my numbers way off?

Recoil is bison balistics
Ab g7's
Hornady calculator 10mph wind 7000ft default environment

6.5prc 153.5 berger .349g7 (or156 or 150smk) 2850fps
Less recoil
Brass and ammo on the shelf
Better barrel life
Same wind drift at 1000yrds
More bullet to do more work
Only gives up ~125fps at 1000yrds


.25-7prc 133berger .311g7
3150fps


6um 115dtacnr .292g7 3250fps

 

[Formidilosus]

I was looking to order up either a 6um or .25-7prc 20"barrel but when I start crunching some numbers I have to ask if there is an advantage over 6.5prc? Am I missing something, Are my numbers way off?

Recoil is bison balistics
Ab g7's
Hornady calculator 10mph wind 7000ft default environment

6.5prc 153.5 berger .349g7 (or156 or 150smk) 2850fps
Less recoil
Brass and ammo on the shelf
Better barrel life
Same wind drift at 1000yrds
Only gives up ~125fps at 1000yrds

A couple of things. One, using Hornady’s max load for the 6.5prc, to get that velocity with 153-156 get bullets, you are a grain low in powder. The 6UM is about 2 ft-lbs less recoil than the 6.5 PRC with 115gr DTAC’s. Shooting identical rifles side by side one 6.5 PRC and one 6UM, the UM feels closer to a 6.5CM in recoil. There is a difference, though small.
Also, your powder and MV is low on the 25-7PRC; you’re looking at 73-74 grains of powder, and quite a bit faster. Wind drift is nearly identical. The 6UM gets about 130-150 yards more of terminal velocity.


But, none of those are why the 6UM specifically. Mainly it’s danger space for the amount of recoil. And that’s at 3,250fps MV which is on the low side- most are at 3,350fps or so. Still, the danger space is better on the UM, with just enough less recoil to be noticeable.

 

[eoperator]

Thank you for the reply.
Updating charge weights and velocities to your numbers on bison recoil calculator with 9lb rifle.
6.5prc 18.7ftlb
6um 21.3ftlb
.25-7prc 27ftlb

I understand these are just calculated estimates but it sure sucked the wind out of my sail for investing in a new build. I was honestly expecting a much larger performance gain/recoil savings.

 

[Formidilosus]

Thank you for the reply.
Updating charge weights and velocities to your numbers on bison recoil calculator with 9lb rifle.
6.5prc 18.7ftlb
6um 21.3ftlb
.25-7prc 27ftlb

I understand these are just calculated estimates but it sure sucked the wind out of my sail for investing in a new build. I was honestly expecting a much larger performance gain/recoil savings.

Something is wrong in your inputs for recoil. It’s not a lot different, but 6.5 PRC does recoil more than 6UM when loaded to max/same.


Shooter recoil calculator:

6.5 PRC is 18.7 ft-lbs-





6UM with 115gr is 17.4 ft-lbs





Now 6UM with 105gr is 16.6 ft-lbs-




The 22UM with 88gr is 14.7 ft-lbs




Again, depending on use the 6UM may not be all that much better than the 6.5 PRC. But depending on use, it is measurably and noticeably better than 6.5 PRC. I, and others have shot them side by side to beyond 1,200 yards. All have had higher hits rates at all ranges and have preferred the 6UM. Most have, or are getting their 6.5 PRC’s rebarreled into 6 UM’s.
Now, again the primary purpose for the 6UM was to maximize hit rates with 6.5cm level recoil, max terminal range, and specifically max danger space.

Danger space was the entire reason I got Ryan thinking about a really fast .224 or 6mm. The 22 version is still my goal and preference.



All at 5,000ft standard conditions and a 10” target:


6.5 PRC and 153gr at 400 yards has Danger space of 123 yards-




6UM with 115gr at 3,250fps, at 400 yards has a danger space of 169 yards-




6 UM with 105gr at 400 yards has a danger space of 192 yards-




That’s quite a bit of difference, and depending on use and user knowledge/skill can allow someone to do some interesting things for animals that move a lot.

Now, the real difference is with something like a 22 UM. The Danger Space for it at 400 yards is 473 yards-




THAT is a very large difference.

 

[Brandon.miller.4]

Can you elaborate on what danger space is? Are we talking maximum amount of error and still impact a 10" circle?

 

[eoperator]

Something is wrong in your inputs for recoil.

My inputs are right tried them again on bison.
I tried several different recoil calculators they all come up with different numbers for the same inputs.

 

[Formidilosus]

My inputs are right tried them again on bison.
I tried several different recoil calculators they all come up with different numbers for the same inputs.

Apologies, I didn’t mean that what you put in was wrong, just something is funky with that calculator and inputs. There is not much difference in ft-lbs of recoil between the 6.5 PRC with the heaviest bullets loaded max, and the 6UM with 115’s loaded max. However, in shooting you can tell which is which.

 

[Formidilosus]

Can you elaborate on what danger space is? Are we talking maximum amount of error and still impact a 10" circle?

Correct. Say you are dialed for 400 yards, it’s how much closer than 400y and how much farther than 400y you can be and still get a hit.

 

[Brandon.miller.4]

Correct. Say you are dialed for 400 yards, it’s how much closer than 400y and how much farther than 400y you can be and still get a hit.

Thats what I inferred. Thank you for the clarification!

 

[BjornF16]

I’m fighting the urge for a 7 PRC because of the .777 G1 with 180 ELD-M:



Only 108 yds of Danger Space with significantly more recoil…

Any advantage of 6.5 SAUM Improved shooting 156 EOL?

 

[eoperator]

I’m fighting the urge for a 7 PRC because of the .777 G1

180eldm BC must change alot with velocity.
Hornady
G1.796
G7 .401

Ab
G1 .703
G7 .360

 

[chrisdixon1]

Curious. What app are y’all using that outputs danger space? That is a very interesting data point that is new to me.

 

[BjornF16]

180eldm BC must change alot with velocity.
Hornady
G1.796
G7 .401

Ab
G1 .703
G7 .360

All bullets vary with velocity. Sierra and Hornady both publish their values.

7mm 180 gr. ELD Match
1 in 8.75" Twist
Mach 2.25
0.777 G1
0.391 G7

Mach 2.0
0.748 G1
0.376 G7

Mach 1.75
0.731 G1
0.368 G7

7mm 180 gr. ELD Match
1 in 7.5" Twist
Mach 2.25
0.816 G1
0.411 G7

Mach 2.0
0.812 G1
0.408 G7

Mach 1.75
0.782 G1
0.394 G7

Ballistic Coefficient - Hornady Manufacturing, Inc

A-Tip®, ELD® Match, and ELD-X® Ballistic Coefficient Values Hornady originally published “800 yard”* average Ballistic Coefficient (BC) values for ELD-X and ELD Match bullets. This was done to provide the most usable BC from a trajectory prediction st...

www.hornady.com

 

[BjornF16]

Curious. What app are y’all using that outputs danger space? That is a very interesting data point that is new to me.

JBM Ballistics online allows you to select “Danger Space” as an output. The iPhone app does not include that option.