6 UM or 6-7PRC?

[Ryan Avery]

For the record, I talked to Blaine on Thursday about a 6mm barrel and he said he had to help you neck those down.

Yes.. he had to help me crush necks. HAHA!

We didn't know if we could neck them down in two steps or three, so we lost a few soldiers.

 

[Ryan Avery]

I did some load dev yesterday.

A couple of takeaways.

We had to size it down in three steps. Because of the necking down process, the COAL was all over the map, making fire forming a must. Once the brass was FF, this rifle was very accurate.

It takes three more grains of 570(69) to match the velocity of a 6UM.

When we average out the speeds of this 20" 6-7PRC barrel and three other 20" 6UM 20" barrels, the 6UM and the 6-7PRC are pretty much ballistic twins. However, a few more 6-7PRC 20" barrels would give us better data.

115 NR DTACS
ADG Brass

570
Three shot average
70g 3370 HOT!
69.5g 3315 Hot-ish
69g 3290 shootable but on the edge
68g 3230 easy peasy

Retumbo
68g 3396 HOT!
67g 3320 HOT!
66.5g 3315 HOT-ish
66g 3260 warm
65g 2195 easy peasy

565
64g 3161 HOT!
63g 3142 HOT!

 

[Jimbee]

I did some load dev yesterday.

A couple of takeaways.

We had to size it down in three steps. Because of the necking down process, the COAL was all over the map, making fire forming a must. Once the brass was FF, this rifle was very accurate.

It takes three more grains of 570(69) to match the velocity of a 6UM.

When we average out the speeds of this 20" 6-7PRC barrel and three other 20" 6UM 20" barrels, the 6UM and the 6-7PRC are pretty much ballistic twins. However, a few more 6-7PRC 20" barrels would give us better data.

570
Three shot average
70g 3370 HOT!
69.5g 3315 Hot-ish
69g 3290 shootable but on the edge
68g 3230 easy peasy

Retumbo
68g 3396 HOT!
67g 3320 HOT!
66.5g 3315 HOT-ish
66g 3260 warm
65g 2195 easy peasy

565
64g 3161 HOT!
63g 3142 HOT!

Peterson brass?
115 dtacs?
Hot=heavy bolt lift, ejector marks?
I'm waiting impatiently to hear from UM on my barreled action.
Thanks

 

[Ryan Avery]

Peterson brass?
115 dtacs?
Hot=heavy bolt lift, ejector marks?
I'm waiting impatiently to hear from UM on my barreled action.
Thanks

I added the components

Yes heavy bolt lift and ejector marks

 

[JBahr]

Is the lack of velocity because of the lack of pressure? At first glance doesn't seem worth the trouble.

 

[snopro176]

Is the lack of velocity because of the lack of pressure? At first glance doesn't seem worth the trouble.

That's what I'm seeing too, long action, 3 part neck down, +3 gr to make it do what a 6UM does? It's hard to get excited about a long action at all these days lol

Sent from my SM-S911U using Tapatalk

 

[Axlrod]

That's what I'm seeing too, long action, 3 part neck down, +3 gr to make it do what a 6UM does? It's hard to get excited about a long action at all these days lol

Sent from my SM-S911U using Tapatalk

I went with a 6 on the 7PRC case because
Brass availability
Dies: Horn. 7 PRC bushing dies are avail now @$120
6 UM dies are backordered $450
Used a Tikka- only one length available.
3 passes through a bushing die is a few extra minutes and only needs to be done once.

That said, when the 6 UM brass shows up I will probably build a 22 UM on a short action like a Bat Bumblebee.

 

[GoatPackr]

so what powder will you settle on?

Kris

 

[Ryan Avery]

Is the lack of velocity because of the lack of pressure? At first glance doesn't seem worth the trouble.

It’s hitting pressure. We are asking a lot of the brass to push that much powder down a small diameter hole. Probably at the point of diminishing returns.


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[Ryan Avery]

so what powder will you settle on?

Kris

570


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[Ryan Avery]

I went with a 6 on the 7PRC case because
Brass availability
Dies: Horn. 7 PRC bushing dies are avail now @$120
6 UM dies are backordered $450
Used a Tikka- only one length available.
3 passes through a bushing die is a few extra minutes and only needs to be done once.

That said, when the 6 UM brass shows up I will probably build a 22 UM on a short action like a Bat Bumblebee.

I agree with all of that, brass availability being the driver. But add in you are going to be FF brass either way and UM should have 6UM dies for around $250 in stock soon.

 

[TaperPin]

I really like these speedy 6mm’s! 7 mag recoil out of a 6mm is pretty cool. Makes my little 6.5 PRC look like a push over.

 

[T_Widdy]

I can guarantee you a fast 6mm feels nothing like shooting a 7 mag.

I really like these speedy 6mm’s! 7 mag recoil out of a 6mm is pretty cool. Makes my little 6.5 PRC look like a push over. View attachment 686719

 

[DangerRanger]

Do you think the straight walls and 40 degree shoulder of the 6um are actually hiding pressure that you’re seeing in conventional signals from the prc case?

 

[GoatPackr]

Do you think the straight walls and 40 degree shoulder of the 6um are actually hiding pressure that you’re seeing in conventional signals from the prc case?

Interesting thought. Much like a Sherman cartridge hides pressure.

Kris

 

[Formidilosus]

Do you think the straight walls and 40 degree shoulder of the 6um are actually hiding pressure that you’re seeing in conventional signals from the prc case?

Absolutely.

 

[JBahr]

Absolutely.

Are we concerned about “hiding pressure” if it doesn’t result in a hindrance/failure of any kind?

To clarify… I tagged in on this thread and others with interest in a fast 6mm. I already have a 6.5 PRC and a 6-6.5 PRC seems like the easy button without having to make the investment in new brass, dies etc. Will I be fighting clickers?!?!

If there is no downside to “hiding pressure” is it worth it to spend some more green and avoid hassle?

 

[wind gypsy]

Are we concerned about “hiding pressure” if it doesn’t result in a hindrance/failure of any kind?

To clarify… I tagged in on this thread and others with interest in a fast 6mm. I already have a 6.5 PRC and a 6-6.5 PRC seems like the easy button without having to make the investment in new brass, dies etc. Will I be fighting clickers?!?!

If there is no downside to “hiding pressure” is it worth it to spend some more green and avoid hassle?

The only downside I can think of to "hiding" pressure is only a downside if you run at high pressure thinking everything is grand and then some debris, moisture, etc comes into the picture and all of a sudden you're piercing primers or worse. Run it a fair bit slower than max where you "see" pressure and i don't see a downside.

My limited experience with cartridges that hide pressure (mostly 6.5x47 Lapua) is that I start to see velocities that tell me it must be hot well before i see pressure signs. It's nice to be issue free into pressures well above SAAMI max.

Whether the 6UM existing reamer and formed adg brass will result in less clickers than an unspecified 6-6.5 PRC chamber/brass combo is probably hard to say?

 

[Ryan Avery]

The only downside I can think of to "hiding" pressure is only a downside if you run at high pressure thinking everything is grand and then some debris, moisture, etc comes into the picture and all of a sudden you're piercing primers or worse. Run it a fair bit slower than max where you "see" pressure and i don't see a downside.

My limited experience with cartridges that hide pressure (mostly 6.5x47 Lapua) is that I start to see velocities that tell me it must be hot well before i see pressure signs. It's nice to be issue free into pressures well above SAAMI max.

Whether the 6UM existing reamer and formed adg brass will result in less clickers than an unspecified 6-6.5 PRC chamber/brass combo is probably hard to say?

I have built so many rifles with AI cases that I don't see a downside on higher pressure in a quality action.

As for clickers, 2 Thou more on the 200 line goes a long way in solving this problem. All of UMs reamers have addressed this.

 

[HighUintas]

What kind of neck diameter, freebore diameter, and bullet jump do you use in the 6/7prc and 6um? I'm wondering if you did anything specifically to tame pressures a bit to allow best case fill.