Short freebore, higher pressure?

[lazyMlazyK]

I've got a 6 Dasher that has recently started showing some signs of pressure. Shooting Alpha OCD brass, 108 Berger BT Target bullets, 30.7 grains of Varget, with CCI 450 primers. Has been grouping very well for me over the summer. I developed the load in roughly 45*F weather, if memory serves me right. Haven't seen any issues until the other day. I went to test my load at 850 yards, and the weather was about 90*F and humid. I haven't shot yet when it's been this hot out, but I had some time and wanted to shoot. Between every twelve shots I'd give the barrel about 20 minutes to cool back off as much as it could, considering ambient temps. I started to see some pretty heavy ejector swipes and even some brass "slivers" that came off at times around where the ejector mark was.

The chamber has a .104 freebore, which I believe is fairly short for running much above a 105gr bullet, as I'm lead to believe. If I were to have the freebore deepened, say, to .120, I'd be able to seat the bullet out about .016" further. Do you think this would help to alleviate some of the pressure I've seen in warmer temps? Or should I just drop the powder charge weight a bit? I'm really not too high on the velocity side of things right now, only seeing about 2835fps, but it shoots pretty good as I have it tuned for now.

Thanks for your replies!

 

[swd]

That’s a light load in a Dasher. How many rounds though the barrel? I’d be looking for carbon build up or a carbon ring.

 

[lazyMlazyK]

That’s a light load in a Dasher. How many rounds though the barrel? I’d be looking for carbon build up or a carbon ring.

There about 400 rounds down the barrel now. I just removed the little bit of carbon ring that had started building up before this shooting session.

 

[fkimble@charter.net]

Hmmm? Maybe double check your powder measure. Make sure it is zero'd when empty. I usually weigh a Sierra 52 match bullet before starting a long powdering session as a sanity check. Was the load worked up in the Alpha brass? Have you recently opened a new can of powder with a different lot#? Maybe drop the load a little for hot weather. How many shots before you noticed the pressure signs? Was the ammo out in the sun or kept shaded til fired? Possible your load is right on the edge.

Frank

 

[RussellJ]

I'm using IMR-8208XBR(30.86 grains) powder and 105 grain A-Max bullets in my 6mm Dasher. I use a 2 Gram Test Weight(30.86 grains) to check my scale.
Doing the same test weight check(3 grams) for my 6XC rifle.

 

[William Vogel]

I've got a 6 Dasher that has recently started showing some signs of pressure. Shooting Alpha OCD brass, 108 Berger BT Target bullets, 30.7 grains of Target, with CCI 450 primers. Has been grouping very well for me over the summer. I developed the load in roughly 45*F weather, if memory serves me right. Haven't seen any issues until the other day. I went to test my load at 850 yards, and the weather was about 90*F and humid. I haven't shot yet when it's been this hot out, but I had some time and wanted to shoot. Between every twelve shots I'd give the barrel about 20 minutes to cool back off as much as it could, considering ambient temps. I started to see some pretty heavy ejector swipes and even some brass "slivers" that came off at times around where the ejector mark was.

The chamber has a .104 freebore, which I believe is fairly short for running much above a 105gr bullet, as I'm lead to believe. If I were to have the freebore deepened, say, to .120, I'd be able to seat the bullet out about .016" further. Do you think this would help to alleviate some of the pressure I've seen in warmer temps? Or should I just drop the powder charge weight a bit? I'm really not too high on the velocity side of things right now, only seeing about 2835fps, but it shoots pretty good as I have it tuned for now.

Thanks for your replies!

I guessing your autocorrect spelled Target instead of Varget powder? 30.7 should be okay.

 

[JLT]

That’s a light load in a Dasher. How many rounds though the barrel? I’d be looking for carbon build up or a carbon ring.

This, right here. Carbon build-up in the throat might very well be the problem. No way less than 31 gr. of Varget behind any 105-108 class bullet is too heavy a load. As pointed out by swd, this is a very modest load in a Dasher, which may have contributed to carbon build-up.

 

[lazyMlazyK]

I guessing your autocorrect spelled Target instead of Varget powder? 30.7 should be okay.

Haha, yup, you got me there. Thanks, I'll correct it.

 

[lazyMlazyK]

This, right here. Carbon build-up in the throat might very well be the problem. No way less than 31 gr. of Varget behind any 105-108 class bullet is too heavy a load. As pointed out by swd, this is a very modest load in a Dasher, which may have contributed to carbon build-up.

That's strange to me though, as I thorghouly cleaned the chamber and barrel and borescoped my work before hitting the range.

 

[ebb]

Sounds to me like a winter load shot in the summer. Ive never had that work I always got pressure when the temp went up.

 

[JLT]

That's strange to me though, as I thorghouly cleaned the chamber and barrel and borescoped my work before hitting the range.

Something is messed up somewhere, then, because there's no way less than 31 gr. of Varget behind a 105-108 gr. bullet will pressure out in a Dasher in Alpha brass. Alpha brass capacity is a shade less than fire formed Lapua, but nowhere near enough to offer up a reasonable explanation.

 

[Sandstorm]

How far in the lands are your rounds. Have you always been shooting the same measurements. ( BTO )
I agree with JLT you should not be having issues.

 

[6fatrat]

The 108 berger is a completely different bullet than the 105 berger.
That combined with your shorter freebore and the heat is why you had pressure problems, imo.
Steve Bair

 

[rhovee]

Just for advice. I worked up a load with 108’s in a dasher recently with Varget in Alpha brass. 32.3 for 2950 in a 28” barrel shoots great and is it near pressure. Really anything from 32.2-32.5 shot “good”.

 

[LRPV]

I do not think a 104 Dasher is long enough for 105 class bullets except 105VLD and I’d still want something longer. I run a 115FB in my Bench guns and it’s shorter than most I’ve heard of people using. Different lots of powder can vary greatly but I’ve ran Varget well over 33 grains in my Dashers before without issue. I would suspect a missed carbon ring, fat bullets (possibly a new lot) causing a lack of neck clearance. Possibly the brass has grown too long and bottoming out in the chamber? Lube left on cases or solution/oil left in chamber? ALPHA OCD brass is hard stuff.