Chamber math

[SDHNTR]

Building a 6GT. Will soon be sending the barrel off for chamber cutting. Primary bullet choice would ideally be a Hammer HHT 90gr, because CA. Although I would also like the flexibility to shoot other bullets, say a 108 Berger Elite Hunter.

With the bullet seated so that the start of boattail is at neck/shoulder junction of case, the Hammer measures 1.960 cbto. I was going to have the chamber cut to allow .040-50” jump as that tends to be the sweet spot for that bullet in my experience.

The 108 Berger Elite measures 1.940” seated in the same manner. So correspondingly, that would mean the jump on the Berger would be .060-.070”. Is that gonna be too much? My only experience with Berger’s is the VLDs and I load them with minimal jump as that’s what they seem to like. I have no experience with the Elites.

Thoughts? Am I painting myself into a corner here? Any other way I should approach this?

 

[waspocrew]

It’s tough to get the throat exactly right for different projectiles. In this case, I’d go a little shorter on the throat for the Hammers. I don’t think you necessarily need to jump .040-0.050 like a Barnes. I’ve set my hammers all at 0.020” off (22 CM, 7 SAUM, 338 RUM) and they’ve shot very well.

That being said, the bergers may surprise you even at the longer jump. Factory Berger ammo for my Seekins 6 Creed didn’t seem to care that it was jumping .120”.

 

[gbflyer]

Shorter. It will get out there soon enough.

 

[WKR]

Building a 6GT. Will soon be sending the barrel off for chamber cutting. Primary bullet choice would ideally be a Hammer HHT 90gr, because CA. Although I would also like the flexibility to shoot other bullets, say a 108 Berger Elite Hunter.

With the bullet seated so that the start of boattail is at neck/shoulder junction of case, the Hammer measures 1.960 cbto. I was going to have the chamber cut to allow .040-50” jump as that tends to be the sweet spot for that bullet in my experience.

The 108 Berger Elite measures 1.940” seated in the same manner. So correspondingly, that would mean the jump on the Berger would be .060-.070”. Is that gonna be too much? My only experience with Berger’s is the VLDs and I load them with minimal jump as that’s what they seem to like. I have no experience with the Elites.

Thoughts? Am I painting myself into a corner here? Any other way I should approach this?

What's your max mag length?
And coal with each bullet?

 

[SDHNTR]

What's your max mag length?
And coal with each bullet?

Mag length is not an issue. Short action. .308 family mag. I’ll never even get close.

 

[WKR]

Mag length is not an issue. Short action. .308 family mag. I’ll never even get close.

So what's preventing you from seating the Berger further out closer to the lands? Would you not have sufficient bearing surface in the neck if you went +.030 further out with the Berger?

 

[SDHNTR]

So what's preventing you from seating the Berger further out closer to the lands? Would you not have sufficient bearing surface in the neck if you went +.030 further out with the Berger?

Borderline. Maybe I could probably seat out about that much, but much more and I don’t think I’d have enough bearing surface in the neck.

 

[WKR]

Borderline. Maybe I could probably seat out about that much, but much more and I don’t think I’d have enough bearing surface in the neck.

I would do some measurements and see what the numbers tell you. Additionally you could make a dummy with the Berger seated out long, then simulate some rough handling and feeding and see if the thing moves. If not I'd say your good to go.

Does the GT have a super short neck length?

 

[Flyjunky]

I went through this dilemma while building my latest rifle. I’d personally go shorter, like @gbflyer said, the jump will only get longer the more you shoot it.

 

[khuber84]

There are two standard 6gt chambers, 0.120 and 0.169 freebore. I've had 9 6gt barrels, only a couple were 0.120 fb. Go long........I jump 108s 40-50k regularly in on my 6cm barrels and that's where they have shot the best.

 

[SDHNTR]

There are two standard 6gt chambers, 0.120 and 0.169 freebore. I've had 9 6gt barrels, only a couple were 0.120 fb. Go long........I jump 108s 40-50k regularly in on my 6cm barrels and that's where they have shot the best.

That’s helpful. How much jump did you have with the .120 on the Berger 108’s?

 

[khuber84]

That’s helpful. How much jump did you have with the .120 on the Berger 108’s?

Never ran 108s in short freebore chambers, just 105 hyb. They have really short bearing surface. I jumped them 30k. By end of barrels life they were jumping around 150k.

 

[SDHNTR]

Never ran 108s in short freebore chambers, just 105 hyb. They have really short bearing surface. I jumped them 30k. By end of barrels life they were jumping around 150k.

Yeah, you’re right about that short bearing surface. Did you see any accuracy degradation as the throat eroded, and jump increased? Or did you chase the lands?

 

[khuber84]

Yeah, you’re right about that short bearing surface. Did you see any accuracy degradation as the throat eroded, and jump increased? Or did you chase the lands?

Left em where they were and just upped powder a lil by lil to keep around 2900-2920. Did the same with 109s in the long freebore chambers.

 

[khuber84]

Keep in mind, I'm not pursuing absolute micro accuracy, as long as it shot half moa or better I didn't mess with anything other than powder to keep the same speeds.

 

[SDHNTR]

Keep in mind, I'm not pursuing absolute micro accuracy, as long as it shot half moa or better I didn't mess with anything other than powder to keep the same speeds.

Roger. Thanks. I’m of the same mindset. I only pursue hunting accuracy.