Interesting seating depth results. 6.5CM Tikka

[Kelly_AU]

I plan to work towards a ladder test for my 6.5CM loads but wont be starting with the bullet in the lands because the bullet would barely be in the case. So I decided to follow Bergers advice and try several different seating depths 40 Thou apart, they are the groups A,B,C,D in the attached pic. corresponding to jumps of 210/170/130/90 Thou. At 90 thou the rounds are at an OAL of 2.925" and are on the verge of magazine feeding issues so shorter jumps are not viable. At 90 Thou there is .1265" of bearing surface engaged in the neck.

Now onto those results ... can anyone offer any advice as to why the 90 Thou group shot in a completely different position compared to all the other groups? The group size at 90 Thou and 130 Thou both were reasonably tight at .270" (100m). I'm trying to decide whether I should proceed with load development at 90 Thou Jump or 130 Thou jump. I previously did a charge weight test to arrive at 42.4 Gr powder as a basis for this test but I will revisit the charge weight via a ladder test in the near term.

 

[clcustom1911]

This is great info. You shooting a CTR? TAC A1?

I'd probably go with the .130 jump because you have more bullet in the case neck and there's less chance of it being knocked out of concentricity from some mild rough handling.

As far as point of impact shift, there could be some starting pressure curve difference with different amounts of jump, etc.

 

[straightshooter1]

I plan to work towards a ladder test for my 6.5CM loads but wont be starting with the bullet in the lands because the bullet would barely be in the case. So I decided to follow Bergers advice and try several different seating depths 40 Thou apart, they are the groups A,B,C,D in the attached pic. corresponding to jumps of 210/170/130/90 Thou. At 90 thou the rounds are at an OAL of 2.925" and are on the verge of magazine feeding issues so shorter jumps are not viable. At 90 Thou there is .1265" of bearing surface engaged in the neck.

Now onto those results ... can anyone offer any advice as to why the 90 Thou group shot in a completely different position compared to all the other groups? The group size at 90 Thou and 130 Thou both were reasonably tight at .270" (100m). I'm trying to decide whether I should proceed with load development at 90 Thou Jump or 130 Thou jump. I previously did a charge weight test to arrive at 42.4 Gr powder as a basis for this test but I will revisit the charge weight via a ladder test in the near term.

In answer to your question: As you shorten your jump the more you increase the volume within the case. More volume with the same amount of powder = less pressure = less velocity = lower POI. So, I'd say you reached a point where the pressure in the case is such resulting in enough drop in velocity to get that POI and it appears to also be within an accuracy node.

 

[Kelly_AU]

This is great info. You shooting a CTR? TAC A1?

Glad you think so. TAC A1

In answer to your question: As you shorten your jump the more you increase the volume within the case. More volume with the same amount of powder = less pressure = less velocity = lower POI. So, I'd say you reached a point where the pressure in the case is such resulting in enough drop in velocity to get that POI and it appears to also be within an accuracy node.

Ah, yes, a simple, plausible explanation. Thanks. Normally I would have had the chronograph with me but sure enough, I didn't have it out there today.

 

[BoltActionBrotherhood]

What were the velocity differences?

 

[clcustom1911]

In answer to your question: As you shorten your jump the more you increase the volume within the case. More volume with the same amount of powder = less pressure = less velocity = lower POI. So, I'd say you reached a point where the pressure in the case is such resulting in enough drop in velocity to get that POI and it appears to also be within an accuracy node.

I was trying to say this, but failed ???

 

[Gee Kay]

What were the velocity differences?

she didnt have the chrono with her or forgot to use it. bad girl!

 

[pabrousseau]

changing seating depth or powder charge causes the bullet to exit the barrel at a different point in its vibration pattern. from the looks of those groups it appears to be a very accurate rifle.

 

[6brshooter]

I'd try the 90 thou off, looks like it really wanted to shoot. I doubt 40 thou extra in the nk is gonna make a big difference in retaining min runout, it's a smooth feeding tikka not an over gassed ar10. What dies are you running Kelly?

 

[phlegethon]

Don’t forget that seating depth is altering at least three things: position relative to lands, case volume, and neck tension. People always talk about neck tension as equivalent to neck size but that is only one component. More bearing surface in the neck means more force required to move the bullet.

 

[A&8's]

Try 95 thou off. That'll get you 1/2 the bullet diameter in the neck.

 

[Kelly_AU]

I'd try the 90 thou off, looks like it really wanted to shoot. I doubt 40 thou extra in the nk is gonna make a big difference in retaining min runout, it's a smooth feeding tikka not an over gassed ar10. What dies are you running Kelly?

These groups were shot with unfired brass ( I know people always say new brass isn't as consistent but I haven't found it to be that big a deal) so no dies in this case, just trimmed.

Some experimentation around 90 thou is definitely on the cards.

 

[6brshooter]

Yes and no. Some brass will have dinged case nks, right out of the bag/box. I always run new brass through sizer to straighten out and align case nks to body/casehead.

 

[AKMarty]

I plan to work towards a ladder test for my 6.5CM loads but wont be starting with the bullet in the lands because the bullet would barely be in the case. So I decided to follow Bergers advice and try several different seating depths 40 Thou apart, they are the groups A,B,C,D in the attached pic. corresponding to jumps of 210/170/130/90 Thou. At 90 thou the rounds are at an OAL of 2.925" and are on the verge of magazine feeding issues so shorter jumps are not viable. At 90 Thou there is .1265" of bearing surface engaged in the neck.

Now onto those results ... can anyone offer any advice as to why the 90 Thou group shot in a completely different position compared to all the other groups? The group size at 90 Thou and 130 Thou both were reasonably tight at .270" (100m). I'm trying to decide whether I should proceed with load development at 90 Thou Jump or 130 Thou jump. I previously did a charge weight test to arrive at 42.4 Gr powder as a basis for this test but I will revisit the charge weight via a ladder test in the near term.

Any further results from this testing? I’m ready to shoot a string of varying jumped rounds all at the same powder load. Referencing the PRB articles by Cal using the 147gr ELDMs.

My current jump with Hornady factory ammo is .107” and it shoots 1/2 MOA there. I have room to load longer and stay in the Mag. I’m targeting about .060” jump to give me consistent POI out to .100”.When I get the data from this I will start moving the charge around to tighten up ES and therefore grouping.

 

[918v]

These groups were shot with unfired brass ( I know people always say new brass isn't as consistent but I haven't found it to be that big a deal) so no dies in this case, just trimmed.

Some experimentation around 90 thou is definitely on the cards.

New brass is just as consistent as used brass. It’s only different and often requires a powder charge and/or coal adjustment to match accuracy with used brass.