Pressure Ring Diameter on Berger 109's?

[dnellans]

Hey all - I'm trying to diagnose a problem and could use some help.

Over the summer I was shooting a lot of Berger 109's that was shooting lights out for me from a 6 creed. I'm a PRS competition shooter and was finishing higher than I ever have, felt like the gun couldn't miss at any distance, etc. Load was 109's over 40.7gr RL-16, cci 450's lapua brass @ 3110FPS. I was also playing with that same lot of bullets in a BRA over 29.0gr 4895, cci 450's, lapua brass @ 2850FPS. Both guns would shoot groups in the .3's from a bipod and read bag which for me and my setup is as good as it has ever been. Life was good.

Ran out of that lot of 2000 109's and purchased another set of 2500. Dropped it into both guns and same load, no other componentns in the system have changed, shot same .3's, maybe .4's at 100 yards so figured I was good to go. Pulled out the chronograph to check them and the BRA is shooting 50FPS slower and the Creed is shooting 30FPS slower. I figured oh well, not a huge deal since still shooting small groups at 100 and didn't really need a big difference in zero or anything.

Long story short - at 2 matches now I've had a -hell- of a time making hits on anything smaller than a 2MOA plate consistently at 400 and beyond. First match I thought maybe I just had a really bad match, but it just happened again yesterday and I was burning all the edges on plates with no consistency.

Starting to question my sanity - I started wondering if this lot of 109's could be shooting well at 100 but terribly at distance. I haven't shot them on paper for groups yet (just built a paper stand for 900yard but its windy this morning) to confirm this is what is going on. But I did measure a few of them, thinking about the velocity drop that happened on both guns.

Averaged over 10 - this lot of 109's at the pressure ring are .24315. for comparison i measured some 103 vtacs which are .24370. unfortunately i don't have any of my old lot of 109's for comparison, but a buddy's 105 hybrids are .24360.

I'm wondering if anyone else has measured their 109's to see what their diameter is on the pressure ring, or thinks this is suspicious? I've never had a load shoot well at 100 but bad at distance before, but combined with the sudden velocity change I am grasping at straws for what could be going on.

Any help diagnosing would be appreciated!

 

[Fred B.]

Throat erosion can causes MV drop And bore erosion will have direct effect on accuracy. How many rounds through the barrel?

 

[dnellans]

Yup thought of that and agree - BRA only had about 450 on it at the time and 750-800 on the creed so well within lifetimes of both.

Related note - I initially I thought oh it must be because I cleaned them (every 200-300 rounds after a match) both, but normally they will only drop maybe 10fps and a couple of shots they'll come back up to speed. these are consistently slower after a match's worth of rounds - the SD/ES on both are great though, sub 10 ES for the BRA and 15 on the creed for 5 shot strings.

 

[barefooter56]

Try adjusting your powder charge up in 1/10th grain increments. Just because the CBTO is the same or within .003 . There may be more or less of the bullet from the new lot down in the neck. This can change pressure/velocity and knock you off an accuracy node. General rule of thumb is : new lot of bullets , brass , powder. Drop load da grain and work back up slowly.

 

[LCazador]

Yup thought of that and agree - BRA only had about 450 on it at the time and 750-800 on the creed so well within lifetimes of both.

Related note - I initially I thought oh it must be because I cleaned them (every 200-300 rounds after a match) both, but normally they will only drop maybe 10fps and a couple of shots they'll come back up to speed. these are consistently slower after a match's worth of rounds - the SD/ES on both are great though, sub 10 ES for the BRA and 15 on the creed for 5 shot strings.

I'd be looking at the seating depth since that lot of bullets' base to ogive may have changed when they made the new lot. So try changing seating depth plus or minus .003".

 

[caihlen]

Just out of curiosity, what tool are you using to get to .xxxxx?

 

[dnellans]

a mitutoyo 1 inch mic - 0000x where x is only 0/5 like most

 

[dnellans]

To follow up on my own question

I measured yet another lot of 105’s and found them to be 2439-2340 making well over 5 tenths between lots of nominally 243 bullets

I decided to call berger and ask the horse directly. The answer is that -within- a lot they QC diameter to within .0003, which matches my measuring. Across lots they make -no-guarantees, which we all know.
When I described the measurement range I’ve seen across bullets and lots as just more than I might expect the person helping me didn’t seem particularly surprised or concerned.

So short answer is that I better start buying in yet larger quantities to not have to change anything mid season.

 

[coford]

Just for fun I measured a box of 109's and three different lots of 105's I have on hand. I measured them all at the boat tail junction and the body of the bullet.

109 PO1786 .2431 at boat tail - .2429 at the body
105 PO1388 .2432 at boat tail - .2429 at the body
105 PO 394 .2434 at boat tail - .2431 at the body
105 #5468 .2432 at boat tail - .2432 at the body

I measured three random bullets of each lot and was surprised the measurement was the same on each three within that lot. My Starrett micrometer goes five places - FYI

From my small sampling it appears that the older lots were a little bigger in diameter than the newer ones.

My personal experience has been that as barrels wear out the fatter bullets seem to shoot better - which only stands to reason.

 

[WVRJ]

You might be experiencing something like a tight barrel vs a loose barrel.Production barrels can be of a slightly different diameter,depending on the tooling used on them.Possibly there's something different in the jacket thickness or the diameter itself that caused the velocity loss.Someone else mentioned bumping the powder to see what happens,and I'd probably think about doing that.Possibly a different machine was used on this lot of bullets and there's a dimensional difference.In these times of ammunition shortages,I'm sure all the companies are running their machines long and hard to try to meet the demand.There's been a general consensus that bullet quality is off for most of the manufacturers because they're pushing hard to make enough.

 

[a-dog]

Are you sure you are using the same lot # of powder?

 

[powderbrake]

Different lots of bullets= start over to find load and CBTO (it will be similar to the old, but mot identical)

I doubt if it is related to the pressure ring.
Check the bullet base to ogive, it changes with every lot.
I suggest using the old load, and adjusting the CBTO in and out in .003 increments from your old settings, and see what the target says.