90 degree crown?

[Nomo4me]

I'm trying to sort out accuracy on a couple of my guns and the crowns have my attn at present.
A local 'smith has finished them with 90 degree crowns.
These are razor sharp at the bore's edge and countersunk with a 1/16" deep 'step' about 3/16" out from the bores edge. I know he indicates them in on a stud so I'm satisfied they are concentric.

Is a 90 degree crown desired? My most consistently accurate varmint gun is a Tikka 595 with what looks to be an 11 degree crown.
I'm thinking of taking both of these guns back to my 'smith and having him recut to 11 degree crowns just for my own piece of mind but he does a fair amount of long range shooting and 90 degrees is what he likes.
Both are heavy bbl guns with low round counts and both are shooting about 3/8 moa bigger than I think they ought to be.

Thoughts?

 

[camac]

Supposedly the 11 degree crown is more accurate but I have seen a lot of exceptional rifles with 90 degree crowns too. I cut a new 11 degree crown on my 6.5 barrel today and they are definately trickier. With a 90 degree crown you can get away with it if you aren't spot on with your concentricity. No such forgiveness with 11 degree. You have to get it right. You may see some improvement by simply lapping the crown. Use either a marble or better still a brass rod with rounded end ( I think Brownells sell them) in drill and fine lapping compound

 

[BoydAllen]

90 degree crowns are probably more the rule in short range Benchrest than 11 degree. They work fine. To keep a rifle at peak accuracy, having the crown recut a couple of times in the life of a barrel is a good idea.

 

[Nomo4me]

Thanks for the replies. One of the guns is a 6 x 47 fitted with a Holland brake. I've pulled the break and will try the most promising loads with it again just to play with the variables.

Best load tried yesterday was 107 SMK pushed by 39 grains H-4350 at 3020 fps which gave me a 7/8" group at 200 yds yesterday. I'd like to be able to expect 1/2" groups from it at 200.

I shot this off a Harris bipod and rabbit ear bag. Sounds like I'll be waiting for my SEB Neo for another year (44th on the list for one).

This gun is also decidedly muzzle heavy - balancing 3.5" in front of the receiver.

 

[Rust]

Angle of the crown doesn't matter, only the precision with which it has been cut. Years back experimentation was done with various crowns, as long as it was as close to perfect as possible, the type didn't matter. The primary reason for the inset is to protect the end of the bore from dings.

 

[BoydAllen]

If you are not using them, with your goal, you need a set of flags, and to tune at the range. Good luck

 

[Scott Harris]

Angle of the crown doesn't matter, only the precision with which it has been cut. Years back experimentation was done with various crowns, as long as it was as close to perfect as possible, the type didn't matter. The primary reason for the inset is to protect the end of the bore from dings.

That's exactly what my gunsmith told me. He suggested using 90 degree crowns because they are easier to cut correctly. I get all mine as 90 degree with a protective recess.

 

[Nomo4me]

I shot my 6 x 47L with the brake removed last week - accuracy wasn't any better = about 1.1" ave at 200 yds. I'm not an experienced BR guy but wanted better results than that.
Before heading out this morning I made the following changes:
- added 1.5 lbs lead to the butt of my McM A-5 to offset the extreme muzzle heaviness
- Switched from 107 SMK's to Berger 108's.
- Arrived at the range at 8:30 am so I could shoot before wind and mirage came up.

My gun:
Pierce single shot action
Krieger 27" bbl with Holland brake
Jewel trigger ~ 4oz
McM A-5 non-adjustable stock
Sightron 10-50 x 60 1/8" dot

In each photo the right hand group is same load as the left, with bullets seated .020" shorter. My RCBS thingie shows my loaded ammo with less than .001" runout.

Groups were shot at 200 yds.

I also tried the 107 SMK's. My gun just doesn't like them, not the way I've loaded them anyway.

 

[camac]

Looking at your targets though it seems as though there is a lot of gain to be had in further load development and technique well before the crown. There are some great groups in amongst it all. The two jammed loads for 108 Bergers are nice (9 fantastic with one horizontal flier - perhaps wind or technique) but a little bit of difference vertically in their position even though only 0.5gr apart -((although 100fps??)). I would suggest you need to explore around there a bit more. Perhaps 39- 40.2 grains in 0.3 gr increments plus fine tune your neck tension / jam jump depth etc.. Feelings are the 6*47 is trickier to tune than a 6BR so keep at it. Don't worry about the crown yet. There are some very good signs amongst all that.
If you still aren't happy with it - you can give it to me..... ;D

 

[Nomo4me]

Thanks Camac. Note the velocity difference is due to different powders - RL-17 vs H-4350 in the second photo.

I was shooting off a bipod rested atop carpet to help fight the bounce. I don't own a good front rest yet.

Those two best groups were kind of exciting for this rank amatuer. If I can replicate them with the same loads that's all the accuracy I need from this gun for the informal long range plinking and prairie dogging I want to do with it.

Looking at your targets though it seems as though there is a lot of gain to be had in further load development and technique well before the crown. There are some great groups in amongst it all. The two jammed loads for 108 Bergers are nice (9 fantastic with one horizontal flier - perhaps wind or technique) but a little bit of difference vertically in their position even though only 0.5gr apart -((although 100fps??)). I would suggest you need to explore around there a bit more. Perhaps 39- 40.2 grains in 0.3 gr increments plus fine tune your neck tension / jam jump depth etc.. Feelings are the 6*47 is trickier to tune than a 6BR so keep at it. Don't worry about the crown yet. There are some very good signs amongst all that.
If you still aren't happy with it - you can give it to me..... ;D