6mm GT chamber question

[StreetCred]

I've got a Tikka T3x action that I'm switching from 6.5CM to 6mm GT. Got the barrel installed. Go/No Go gauges verified the headspace is correct. I created a dummy round to find the jam/lands. I'm using brand new Alpha brass with Hornady 105gr BTHP bullets. The Hornady manual says COL for this should be 2.520". I'm getting 2.472". Measured 5 times with the same results.

Is this normal? Would factory ammo even fit in the chamber? Not new to reloading but I am new to installing new barrels. Just looking for a sanity check. Thanks!

 

[Wrench]

Sharpie a round, chamber it and see for yourself.

 

[StreetCred]

Sharpie a round, chamber it and see for yourself.

That's basically what I did. Didn't color the bullet. I loaded it to the books 2.520". After chambering the round it measured 2.472".

 

[Wrench]

If it pushed back that far, it's probably going to kiss at more like 2.45". I'm surprised that you could get the cartridge out without pulling the bullet if it moved that much.

I would take a bullet and expanded or fired case and put a drop of super glue in the neck, set a bullet in long and chamber the round. Leave it for a few minutes and measure. That's as close to a kiss as you can measure.

In the end, it doesn’t matter what the number is as long as it feeds, extracts whole and holds accuracy. The number is just what you'll record as a reference mark. Your situation sounds like the throat is the limiting factor....which is really no big deal. You can run it as is or ream it.

 

[skipper907]

I would look into what the freebore was cut to. It could have been cut to something under sammi spec

 

[StreetCred]

I would look into what the freebore was cut to. It could have been cut to something under sammi spec

I've got an email into the smith that cut the chamber. I'm assuming it would be cut to SAMMI spec. I wasn't given any options other than "6MM GT".

 

[43.6N]

I've got a Tikka T3x action that I'm switching from 6.5CM to 6mm GT. Got the barrel installed. Go/No Go gauges verified the headspace is correct. I created a dummy round to find the jam/lands. I'm using brand new Alpha brass with Hornady 105gr BTHP bullets. The Hornady manual says COL for this should be 2.520". I'm getting 2.472". Measured 5 times with the same results.

Is this normal? Would factory ammo even fit in the chamber? Not new to reloading but I am new to installing new barrels. Just looking for a sanity check. Thanks!

Make another one, and see if your measurement method is consistent.

 

[skipper907]

I've got an email into the smith that cut the chamber. I'm assuming it would be cut to SAMMI spec. I wasn't given any options other than "6MM GT".

Ya I would see what he has to say and talk it him about the problem you are having.

I know with berger the listed coal for there load data doesnt work with and I think the load data is before the GT was sammi spec'd. On PBB site they list 2 different freebore options.

but it sounds list your freebore is short

 

[JohnsIsl]

Yeah, sounds like your freebore is short. I load BTO at 1.900, which puts a 107 TMK right around 2.662 OAL. This is still at least .030 from my lands. Pretty sure mine is SAAMI.

 

[StreetCred]

I think it has something to do with the shape of the bullet. This is what I found after repeating the process 5 times with each bullet type. I would seat the bullet to 2.660" in a case that has a slip fit ID. It's loose enough that I can start inserting the bullet by hand.

Hornady 105gr BTHP Match (bullet length 1.225"): 5x average of 2.472" COAL

Hornady 109gr ELD-M (bullet length 1.316"): 5x average of 2.590" COAL

Doesn't explain why the 105gr jams before the 109gr. Tight throat?

 

[Coleredding00]

I think it has something to do with the shape of the bullet. This is what I found after repeating the process 5 times with each bullet type. I would seat the bullet to 2.660" in a case that has a slip fit ID. It's loose enough that I can start inserting the bullet by hand.

Hornady 105gr BTHP Match (bullet length 1.225"): 5x average of 2.472" COAL

Hornady 109gr ELD-M (bullet length 1.316"): 5x average of 2.590" COAL

Doesn't explain why the 105gr jams before the 109gr. Tight throat?

Measuring COAL is an exercise in futility. Different bullet profiles will always engage the throat in different ways, and different profiles all have different bullet lengths.

Even measuring base to ogive, I often see a difference between a Berger bullet and a Hornady bullet because the difference in shape Will engage the rifling differently.

Do the lock tite/ super glue trick mentioned above, or buy the Hornady tool. Or just seat them to a round number that’s far enough from the rifling to not jam the rifle up if there’s a speck of dust in the chamber. Chances are it’ll shoot the same either way. Seating depth is an over rated measurement IMO.

 

[Lawnboi]

Different bullet profiles require different coal. Easy check would be to get some saami ammo, though it’s so close I’m not sure I’d dink with it.

I’d go buy a bunch of 105 Berger hybrids, load them .040 off jam and shoot away.

Freebore as it relates to what bullet you want is important. The next one hopefully you will have a better understanding of it.

All measurements should be CBTO when relating your ammo to your chamber. All coal matters for us if it fits in your mag.

 

[wind gypsy]

I think it has something to do with the shape of the bullet. This is what I found after repeating the process 5 times with each bullet type. I would seat the bullet to 2.660" in a case that has a slip fit ID. It's loose enough that I can start inserting the bullet by hand.

Hornady 105gr BTHP Match (bullet length 1.225"): 5x average of 2.472" COAL

Hornady 109gr ELD-M (bullet length 1.316"): 5x average of 2.590" COAL

Doesn't explain why the 105gr jams before the 109gr. Tight throat?

This table is the best visual I can give of why - Look at the nose length column. 95 Classic hunter (probably closest resemblance of 105 Bthp shape) has a nose that is 0.200"+ shorter than the 115 VLD so COAL is going to be significantly different between the two for start of bearing surface to be at lands.

Bullet Reference Charts

DESCRIPTION LEGEND FB: Flat Base High BC FB: High BC Flat Base BR Column: Bench Rest Flat Base BT: Boat Tail Long Range BT/Juggernaut: Long Range Boat Tail VLD: Very Low Drag Hybrid: Combination Secant and Tangent Ogive LR Hybrid: Long Range Hybrid AR Hybrid/Classic Hunter: Short Nose Hybrid...

bergerbullets.com

 

[swavescatter]

I set up my bullet seating die for my 243 and 108 ELDM's for max COAL and was shocked I had to seat 95 grain Nosler BT's so much deeper to avoid jam. Looking at the difference in ballistic coefficient that should not have been a surprise...

Moral of the story: Always measure and verify distance to lands for each bullet. Do not assume that going with a shorter/lighter bullet will be safe.

 

[JohnsIsl]

CBTO should be the measurement used. Someone previously mentioned this.

Here is an example using two 6 GT cartridges. Both have a CBTO measurement of 1.900. The cartridge on the left COAL is .050 shorter than the one on the right.

 

[Lawnboi]

Another example of bullet shape and CBTO/coal. All these bullets are you.020-.040 off the rifling. All three have very different bullet shapes.

 

[StreetCred]

Another example of bullet shape and CBTO/coal. All these bullets are you.020-.040 off the rifling. All three have very different bullet shapes.

View attachment 1072931

That's what I was talking about. The BTHP will jam .050" before the ELD-M. It seems like I have a short freebore where the long, skinny ELD-M can extend into the throat more than the shorter, fatter near the tip BTHP.

I'm fine with seating different bullets to different depths to maintain similar jump to the lands.

Really, my concern is if I'm pushing the BTHP too deep into the case. Am I going to have pressure issues? Is it going to be unsafe?

 

[Lawnboi]

That's what I was talking about. The BTHP will jam .050" before the ELD-M. It seems like I have a short freebore where the long, skinny ELD-M can extend into the throat more than the shorter, fatter near the tip BTHP.

I'm fine with seating different bullets to different depths to maintain similar jump to the lands.

Really, my concern is if I'm pushing the BTHP too deep into the case. Am I going to have pressure issues? Is it going to be unsafe?

Negative deeper in the case likely won’t have any issues with safe loads.

The opposite can cause some pressure. Jamming a bullet in the rifling can cause issues.

I don’t even really read book coal, I measure a new chamber on where my bullet contacts the rifling, measure with a CBTO comparator, and adjust my die to where I want it seated. From there you can experiment.

 

[StreetCred]

I might just be over thinking this too much. I had an issue with a different barrel in 6.5CM that had almost zero freebore. I was getting pierced primers. Actually, blew the bolt shroud off the back of the bolt and into my face. Not a pleasant experience. Do not recommend. So, pardon the pun, I'm a little gun shy about anything being out of the norm.

 

[Lawnboi]

I might just be over thinking this too much. I had an issue with a different barrel in 6.5CM that had almost zero freebore. I was getting pierced primers. Actually, blew the bolt shroud off the back of the bolt and into my face. Not a pleasant experience. Do not recommend. So, pardon the pun, I'm a little gun shy about anything being out of the norm.

Much smarter to verify by measuring rather than blindly believing a reloading manual.