.308 neck tension?

[jbpmidas]

I am trying to work up a load for my first try at 600 yd F-Class. I have a 168 SMK load and 175 SMK load that have performed ok at 100 and 200. Both have given 5 shot groups under 1/2" at 100 and under 1" at 200 from a bipod. However, after that group they are likely to shoot a 1-1/2 to 2 MOA group. We chronographed them today and the ES's were 100+. We measured the loaded brass and resized brass and got a .007" difference. Can too much neck tension cause the ES's I am getting?

Thank you!

 

[jbpmidas]

You would have to have very extreme variance to neck tension to get ES that bad.

Couple thoughts for you:
- Using a "powder thrower" with large granular powders can cause high extreme spreads.
- Mixing brass can cause high extreme spreads
- If the load is between accuracy nodes will cause high extreme spreads.
- Inconsistent seating depths can cause extremes.
- Chronograph is to close to the muzzle can cause powder blast readings. I suggest no less then 10' from the muzzle.

Hope this helps.....
Donovan Moran

The powder charges were weighed on an RCBS 10-10. This batch was loaded in once fired FC brass. I don't know anything about accuracy nodes. These two loads were the best performers from previous attempts. I did not measure every loaded round for length, so I cannot say, 100%, that they were exactly the same length. The chrono was @12' from the muzzle according to our tape measure.

 

[6BRinNZ]

There was another thread on this site asking the exact same question with the same ES (100fps). Chrono miss reading was suggested a lot by posters. I have had 70fps ES due to chrono readings i.e. bright sunlight it was showing 70fps faster than when the sun went down.

 

[MrMajestic]

Wouldn't the good groups have the same high ES? The way I read your post it sounds like fouling or barrel temperature increase is your issue. What rifle? Bedding? Please correct me if I am wrong. You can go to JBM's site and see what 100 fps does to your trajectory but I am betting its not 1-2 MOA.

 

[necchi]

I don't know anything about accuracy nodes.

Accuracy "nodes" are just another way of saying the "Harmonic balance" node, it happens when everything falls in line. 2 even 3 "nodes" can usually be found when testing different powder charge/bullet combinations.
If your going for 600yrd stuff you should school yerself a little bit about these handy places to find accurcy for your gun.
There's a couple ways to find these "nodes"
The Audette ladder;
http://www.6mmbr.com/laddertest.html
Dan Newberry's OCW method;
http://optimalchargeweight.embarqspace.com/

Varmint Al's pages has some really good info bout "harmonic balance";
http://www.varmintal.net/amode.htm

 

[jonbearman]

What powder are you using and primers and bullets for the test that went awry. What twist is the barrel? What length is the barrel and what kind of barrel. I am assuming it is .308 with the weight of bullet you are using.Have you tried the sierra 1555 match kings?

 

[jbpmidas]

Wouldn't the good groups have the same high ES? The way I read your post it sounds like fouling or barrel temperature increase is your issue. What rifle? Bedding? Please correct me if I am wrong. You can go to JBM's site and see what 100 fps does to your trajectory but I am betting its not 1-2 MOA.

I have the chrony data in my range bag. I was pretty disgusted when I got home from the range yesterday and haven't looked at it. I will post some real velocity numbers this evening. I know it is not a barrel fouling issue, but I guess it could be a temperature problem

The rifle is a Remington 700 Target/ Tactical. It came with a 26" 5R VTR barrel. It is in a B&C A5 adjustable stock with aluminum bedding block. I have not bedded the action.

 

[jbpmidas]

What powder are you using and primers and bullets for the test that went awry. What twist is the barrel? What length is the barrel and what kind of barrel. I am assuming it is .308 with the weight of bullet you are using.Have you tried the sierra 1555 match kings?

With the 168SMK's I am using H335 and with the 175SMK's I am using IMR4895. We will be getting some Varget to try in this gun and a Savage F-T/r .223 my shooting buddy just picked up. I want to try the 155 Palma MK's, but have not picked any up yet. I believe I was using Remington primers, however, my next time out I am going to try Wolf LR's. The barrel is a 26", 11.25" twist. It is the 5R VTR from Remington.

I appreciate everyone's help.

 

[Heavies]

I think 335 is too fast.

Try out some Varget with those 168's and 175's, also, try some 8208 with 155's. I've heard 8208 will also work with 168's and 175's, but I havnt tried these yet.

 

[jbpmidas]

I think 335 is too fast.

Try out some Varget with those 168's and 175's, also, try some 8208 with 155's. I've heard 8208 will also work with 168's and 175's, but I havnt tried these yet.

My shooting buddy actually picked up apound of Varget and a pound of 8208 today.

 

[Rtheurer]

JB

I have to tell you Im not a big fan of Federal brass. I have a BUNCH of it but it gets deligated to hunting brass and only loaded 2 or 3 times then I toss it. What is your loaded round Neck Measurment? My palma ammo is .337. I use a .336 neck bushing to size my brass which gives me .001 neck tension. On my hunting ammo I run .002 neck tension do to possable rough handling. You should not need that shooting F class. How often do you need to bump the shoulder back on your brass. After the 3rd firing? Fclass target is a real mother of a tight elevation and you almost benchrest quality ammo to do well to keep it in the 10 ring. 100fps ES sure seems high and I think you can get it alot better. Go to the 308 Guide here and read up on the choices alot of champions use. 43 to 45 grains of Varget, that russian primer you have, very very carefully weighed powder charges and proper shoulder set back and neck tension should give you every thing you need.
Good luck
RussT

 

[spclark]

FC's not the best brass out there for consistent, accurate loads in 308. I've been using 8208XBR this year with great results for both the Sierra 2156 (new Palma) and 190 MK in Lapua brass with no more than 0.002" neck tension.

Primers can have a dramatic effect on SD's, as of course can propellant choice, charge weight and seating depth variation. The Russian / Wolf primers I use are great in that respect; SD's around 5 - 8 whether LR (KVB-7) or SR (KVB-223REM).

When you say "measured brass" are you measuring loaded neck diameter or shoulder position before & after firing? 0.007" is quite a bit in either case.

If your case shoulders are moving this much you need to re-size your cases less or you'll be suffering from case stretch in no time.

If your fired necks are that much larger than your loaded rounds, you need to get a gunsmith to make a Cerrosafe casting of your chamber & get an idea of what your chamber neck diameter is.

 

[MrMajestic]

If you do indeed have 100 fps ES you may have multiple issues. As I stated previously 100 fps ES does not equate to 1-2 MOA. If the good first groups were shot with the same ammo, what changes? Does this happen every time you shoot, good then larger groups? That's what I'm reading. Does this have the triangle shaped barrel? If so is it floated adequately? Sorry for so many questions just trying to help. There is great advice on load data supplied here but may not be all your answers.