Velocity is climbing and I'm getting flyers... can you help?

[diddlehop]

It's a new gun, semi custom 7WSM built on a Savage Accutrigger action and with a stock Savage 26" barrel with muzzle brake.
The action is trued and timed and bedded into a custom fiberglass fingerhole stock.

Doing load development I settled on 63.3 grains of H4831sc, Sierra Game King 160gr bullets in new Winchester cases. (this is a long range hunting rifle) OAL is 2.8". I forgot how much jump to lands, but it's not a lot.

Now the issues. When I first settled on this load it was chronoing about 2970 ish. As I put more rounds through the gun the velocity has climbed up to about 3015 fps. The gun is extremely accurate when I get everything right but I'm worried about this climb in velocity and I'm getting flyers about every 5 or 6 rounds. Total of about 87 rounds through the gun now. I clean about every 15- 20 rounds but have not used a chamber brush or mop yet.

The extra velocity has not hurt the accuracy, if anything it improved. I've been getting sub .5 moa out to 550 yds with 4 shots, then I get a flyer.
Here's the velocities showing the climb, one line to a shooting session...There were more shots made, not all were chronoed.

2912, 2968, 2965, 2970, 2988, 2991, 2993

2970, 2991, 2990, 2989,2982

2997, 2994, 3013

2988, 3001, 3002, 3011, 3017
The first shot of the last group was a flyer.
The last 4 shots yielded .347 moa at 550 yds in the vertical, about .6 moa horizontal because it was windy and gusty and I'm just getting used to the scope with 1 moa windage hashmarks.

I think part of the problem for the flyers is inconsistent neck tension. I use a Redding competition die set and a RCBS digital powder scale. Runout as measured on a Hornady Concentricity tool is generally under .003, most under .002. It was recommended to me to anneal the brass even though it's new. This makes sense to me and I'll try it but I'm curious to know what else to look for. I also turn the necks to remove the high spots.

What would cause the rise in velocity that I've seen?
thanks for your help!
diddlehop

 

[Forum Boss]

It's not unusual for velocities to increase as the rough spots in a barrel, particularly a factory barrel, polish out. I know that is contrary to "science" as less friction should mean lower pressure, but this is an empirically observed phenomenon in some barrels.

Regarding the "flyers", there are a zillion potential causes, including the trigger puller. Inconsistent brass or inconsistent neck tension could be contributing to this, but frankly, I'd put my money on the factory barrel. It is exceedingly rare to find a factory barrel that will consistently hold accuracy for an extended shot string. I had a Rem .260 with a Factory Savage barrel and 4 shots clustered with one out was almost a given with any load, any bullet. When I switched to a custom barrel, voila, "flyer" problem solved. Why do factory barrels shoot better -- lot of reasons. Better steel, stress relief, hand-lapping, more precisely cut crowns, correct throats. We have seen some Savage throats that were reamed off-axis to the bore. One such gun shot surprisingly well, but could never get down below the 6s and 7s.

 

[Kenny474]

I think the Forum Boss meant "why are Custom barrels better".

I have had a few factory Savage barrels, and they didn't really start shooting to their full potential until about the 200rd mark. They all gained velocity as they broke in, and all threw an occasional flier. They got better after they smoothed out a bit, but never to the degree of a custom barrel. You are getting exceptional accuracy considering you retained the stock barrel.
You also may have a little carbon build up, which will cause pressure to rise as well. Hit the barrel with a good carbon solvent and let it soak a bit.

Also new cases tend to be a little less consistent than once fired in my experience. After they get fire formed they tend to get a little more consistent and group a bit better, especially in a factory chamber that is probably a bit on the loose side compared to a match chamber. I would run through the brass you plan to use and then weight sort and neck size it. See if things improve after that.

Few questions as well. Was the crown re-cut after the brake was installed? Did you size the new brass prior to firing? Have you started using once fired brass yet, and do you neck size or FL?

Your not doing bad for a factory tube anyway. Under 1/2MOA at 550 is pretty dang good!

 

[diddlehop]

Boss and Kenny, thanks for taking the time to reply, much appreciated.
I had originally intended to get a custom barrel but it became painfully obvious that I wouldn't get it in time to prepare for hunting season. A custom barrel is still a possibility for next year. But if I can make the stock barrel work I'd prefer to save the dollars.

To answer your questions Kenny474;
I don't know if the crown was recut, yes, I resized the brass before firing. No I haven't started on the fired brass yet, maybe late next week. I plan to full length size the fired brass with a Redding full length body die, then use the bushing die on the neck.
I agree that getting 1/2 moa groups is very encouraging, but for fair chase hunting I have to limit my range to where the groups with the flyers tell me to. I want this gun for clean kills out to about 750-800 yards. No way I can shoot at an animal at those ranges right now until I get rid of the flyers. Unless things improve I'm thinking my limit will be about 450 yards for this season, and then only under ideal conditions.

One more question for you guys... I have never pushed my reloads to the upper limit so I don't have experience detecting signs of excessive pressure. I remember my gunsmith saying that the best way was to monitor growth of the case diameter near the head. Right now I'm getting a good .006 increase after firing the new cases. What do you think? Is that ok, how far can it go?
diddlehop

 

[Kenny474]

Try using brass that is neck sized only before you go and FL size all your brass. I know you are supposed to FL size for hunting loads, but it's worth trying to see if tighter cases will improve your groups. But try chambering a few that are neck sized only before you start loading ammo, that way if there is excessive resistance you don't have to break down loaded rounds. If they don't chamber, then FL size them.
I only neck size for my bolt guns, and FL my semi-autos. When they are difficult to chamber I will run them in the body die, but only enough so they chamber easily, only pushing the shoulder back 1 or 2 thou. A Hornady headspace gauge will be very helpful for setting up your body die. That way you only size the minimal amount.
Getting a custom ground body die may also be an option, as factory chambers can be a little on the large side. That way you are only sizing the minimal amount for reliable chambering, better for group size and brass life.

As for pressure, I usually go by what my primers look like and how hard they extract. If my primers are cratered around the firing pin, I know I'm too hot. If they are flat but not cratered, I'm warm but safe. If they have not flattened or backed out of the case, not hot enough.

Article on reading brass for pressure:
http://accurateshooter.wordpress.com/2007/06/29/detecting-excessive-pressure-in-your-cases/

You could also do a search on the site, as I believe there may be other instances where pressure is discussed.

 

[jb1000br]

I am going to bet carbon build up...especially since you are using more mild loads...higher pressure usually means better/cleaner burn.


JB

 

[bozo699]

I am going to bet carbon build up...especially since you are using more mild loads...higher pressure usually means better/cleaner burn.


JB

I am with JB ,almost everytime I see my guns rising in velocity for no apperant reason I scrub the throat out and they settle right back down.
Wayne.