Velocity change with seating depth change

[P_Mo80]

308, Remi 700, 20 barrel, shooting 178 ELD-M's sitting on 43.3 gr of Varget

I did my initial load development 60 thousandths off the lands. With this recent test, I moved it up to 20 thousandths off the lands. Everything else was kept the same.

Groupings were much better being closer. After adjusting for a 30 degree temp drop from my last test, the velocity dropped 45.1 fps. That sound reasonable for a 40 thousandths change in seating depth?

 

[Tokay444]

Were you compressing powder before?
Normally pressure decreases as you move away from the lands, until you start compressing powder that is.

 

[P_Mo80]

No, wasn't compressing loads. I was thinking that pressure would decrease since I expanded the "space" inside the case by moving the bullet 40 thousandths (longer - towards lands). No?

 

[Tokay444]

No, wasn't compressing loads. I was thinking that pressure would decrease since I expanded the "space" inside the case by moving the bullet 40 thousandths (longer - towards lands). No?

The more of a running head start the bullet has into the lands the less pressure spikes.

 

[supercorndogs]

How many rounds have you chronographed total in both tests? Same lot of bullet? Same brass? Did the brass go from virgin to 1x fired?

 

[P_Mo80]

How many rounds have you chronographed total in both tests? Same lot of bullet? Same brass? Did the brass go from virgin to 1x fired?

20 Rounds total, 10 first test (2541 Avg fps), 10 second test (2484 Avg fps). I had a 30 degree drop in temp, between the two tests, so adjusted by 12.4 fps for powder temp sensitivity. Same lot of bullets. This batch of brass was coming in to both tests as 3rd firings.

 

[P_Mo80]

How many rounds have you chronographed total in both tests? Same lot of bullet? Same brass? Did the brass go from virgin to 1x fired?

You know ... I just realized I switched from an optical chronograph used on the 1st test to a magneto speed 2nd test. Would there be that much of a difference in fps? I had the optical 10 feet from muzzle. Magneto obviously is at the muzzle.

 

[Tokay444]

Definitely a variable.
Shooting the other seating depth over the same chrono is the only obvious test.

 

[TheOfficeT-Rex]

You know ... I just realized I switched from an optical chronograph used on the 1st test to a magneto speed 2nd test. Would there be that much of a difference in fps? I had the optical 10 feet from muzzle. Magneto obviously is at the muzzle.

Yup, that could do it - not only the distance but they very likely wouldn't read the same even if you used them at the same time. Only one way to find out...

 

[P_Mo80]

Yup, that could do it - not only the distance but they very likely wouldn't read the same even if you used them at the same time. Only one way to find out...

Looks like I have some data to go get! Thanks for responding everyone!

 

[ReplicaSnipers]

There

You know ... I just realized I switched from an optical chronograph used on the 1st test to a magneto speed 2nd test. Would there be that much of a difference in fps? I had the optical 10 feet from muzzle. Magneto obviously is at the muzzle.

There's an article on Precisionrifleblog containing an assessment of error in various chronographs. Magnetospeed does very well at low error but nearly all other chronos were terrible. I would say that this would make a massive difference in the recordings.

 

[Barelstroker]

Definitely a variable.
Shooting the other seating depth over the same chrono is the only obvious test.

Just out of interest, what is the general consensus on changing charge weight while keeping to a known good performing seating depth?

 

[Dthomas3523]

Just out of interest, what is the general consensus on changing charge weight while keeping to a known good performing seating depth?

Both achieve same thing. Timing the bullet during the barrel vibrations.

 

[Barelstroker]

Both achieve same thing. Timing the bullet during the barrel vibrations.

Yes, I know that.
Does a well performing seating depth continue to maintain good consistency with change in charge weight though?
For example, have you found when commencing OCW tests on a different powder that a known seating depth performs generally well?

 

[Dthomas3523]

Yes, I know that.
Does a well performing seating depth continue to maintain good consistency with change in charge weight though?
For example, have you found when commencing OCW tests on a different powder that a known seating depth performs generally well?

I don’t don’t do load development that way. I break my powder charge and seating depth into two different portions of testing and don’t intermingle the two as it’s not necessary with modern equipment.

In theory, it would perform well until the charge weight is enough that it throws the load out of “time” with the barrel. This would be more a factor of the barrel (contour and such) than it would anything.

 

[salks]

Yes, I know that.
Does a well performing seating depth continue to maintain good consistency with change in charge weight though?
For example, have you found when commencing OCW tests on a different powder that a known seating depth performs generally well?

You should check out the bullet jump research on the precision rifle blog and the short action custom home page. Some say there is a jump node for a bullet. The only way to be sure is to do some testing yourself. I have never seen an increase of more than say 10FPS changing seating depth and that's -.010 to -.100 with H4350 in 6.5 CM. What you want to do with your load also is a consideration. I just want to find something that works and wack steel. I hate testing anymore.

 

[Tokay444]

Yes, I know that.
Does a well performing seating depth continue to maintain good consistency with change in charge weight though?
For example, have you found when commencing OCW tests on a different powder that a known seating depth performs generally well?

Mark Gordon’s testing leads to yes. He starts his charge weight testing at his known seating node for a particular bullet.

 

[Barelstroker]

Mark Gordon’s testing leads to yes. He starts his charge weight testing at his known seating node for a particular bullet.

Glad to hear that. I haven't done enough tests to be conducive but, I've been working up some loads around Sierra 135 grain HPFB & I keep coming back to around 2.247 for those & Speer 130's. Surprisingly, the best results velocity SD wise have been with the fast burning AR2207 which equates to H4198, Reloader 7 or IMR 4198. I just got my 1st batch of BM 8208 which, I think is the same as IMR 8208 XBR. I've loaded some OCW loads with 8208 seated at 2.247 hoping it will follow the trend.
For years I've just picked a 95%-98% book listed charge weight & tinkered with seating depth & always got reasonable results but, lately, I thought I'd give the OCW a go &, it's been quite promising, except for the time & consumables.

 

[Tokay444]

What types of velocities are you seeing from the 135s with those faster powders.

 

[Barelstroker]

What types of velocities are you seeing from the 135s with those faster powders.

Sorry Tokay, I didn't realize you'd posted. I know it's been some time but, I have the Vels for you.
These were all measured on a Caldwell & recorded on my phone & are the Averages of 5 shot groups.
Powder AR 2207
Charge weight 36 Grns
Bullet 130 grn Speer HPFB
Primer CCI 200
2594
2578
2589
2606
2596
2557

These next 3 are identical except the primer which was changed to Rem 9 1/2
2651
2641
2640

 

[Tokay444]

Sorry Tokay, I didn't realize you'd posted. I know it's been some time but, I have the Vels for you.
These were all measured on a Caldwell & recorded on my phone & are the Averages of 5 shot groups.
Powder AR 2207
Charge weight 36 Grns
Bullet 130 grn Speer HPFB
Primer CCI 200
2594
2578
2589
2606
2596
2557

These next 3 are identical except the primer which was changed to Rem 9 1/2
2651
2641
2640

And that’s in 308 like the op?

 

[Barelstroker]

And that’s in 308 like the op?

Yes 308 Win.
Winchester brass. Howa 1500 in MDT ACC.
The vels that I listed weren't the actual vels I was referring to at the time although they are basically the same. I had to re test the CCI200 loads because they were recorded on my previous phone which I left on my car roof & drove off. Those above were recorded 17/12/2020
The Rem 9 1/2 were very interesting I thought. I've had some pretty good results with those Rem primers regards ES-SD. The Rem primer were 10 shot averages.

 

[Tokay444]

Seems very slow for such a light class of bullet.

 

[Barelstroker]

Seems very slow for such a light class of bullet.

Yeah, 20"barrel, & 2207 is fairly quick. 37 grns is ADI book max & the load I listed above seated to 2.473" were grouping under 1 MOA consistently so I wasn't too concerned with the lowish velocity. Those HP's make a bloody awful mess as it is so no reason to get em going any quicker.

 

[1moaoff]

Pressure will also go up if jam length or close.

Less pressure in case with longer seating.
More pressure at jam..

There's a line there between them

 

[Barelstroker]

Pressure will also go up if jam length or close.

Less pressure in case with longer seating.
More pressure at jam..

There's a line there between them

AR 2207 is the fastest burning powder listed in the ADI reloading manual for that bullet weight. It just builds pressure very early on. The bullet is at least 130 thou away from lands.

 

[GONE BAD]

This is my $.02 FWIW.

I purchased a factory 300WM earlier this year for Elk hunting.
In years past I have used my 6.5-284 with 140gn Bergers VLD-H with great success. It has preformed great. My last kill was 690 yards. 1 shot, DRT. To me 800 yards was the max I would take.
I had a shot at 1150 yards but wouldn't take it. That was at 7am watched it all day at 3pm we decided to try and cut the distance. We were able to make a excellent clean kill.

Ok fast forward to today.

I have spent the last few months with the new rifle in hopes of extending the range out to 1200 yards.
While I probably won't take that shot I still want the confidence that a shorter distance will improve my skill level.

So here comes my testing.
With all the shortages of components it had been a hard journey to say the least.
The only powder I've been able to get in a volume had been imr7828ssc. The bullet choices are equally as scares.
And primers nonexistent.

My loading experience for this caliber is very limited to say the least.
When I first started I was loading near max loads with a 200gn bullet (Bergers)
at mag length 3.50" burned up valuable components and got nothing I would shoot over 300 yards at game.
My velocities were running into the 3000fps but I couldn't get them to shoot!

Then I was able to obtain 500 200gn ELDX and started over.
They were a little better but nothing good. Still at mag length.

My next step was to give up on speed and look at all the data I could find concerning this bullet and it all lead to COAL length of 3.340 and velocities around 2850fps so I knew what powder charge would give that approximate velocity and I loaded up 20 rounds.

This is where it got interesting.
I took a fired case loaded it with my powder charge and slid a bullet in gingerly and checked the OAL.
It was ,give or take, 3.440. So I loaded 10 rounds, not giving up on mag length, to 3.490"
Then I loaded 10 at 3.440"

This is what I found, keep in mind my OAL in this rifle is like .250 jump from mag length of 3.50";
The OAL of 3.490 shot Not good and was at just under 2900fps.

The ones loaded to 3.440 cut the group size in half and right around 2800fps!

So seeing these results I started a new approach.
Still wanting to get this load to at least 2850 and now knowing shorting the OAL
would give me better accuracy while also lowering the velocity, I loaded 20 rounds one gn more of powder, still way under book max and at the same time shortened the OAL to 3.420".

Took these rounds out to the range and shot the best group ever with this rifle using my handloads!
I forgot to bring my MS so couldn't check speed and so only shot 5 rounds.

Not being able to get back to the range for a couple weeks I finally was able to and the results were amazing .
First shot was low and the next 3 were in one hole! Those 3 measured .183"
Including the 1st round it was .67" .
That was also the 1st cold bore shot.

I set the rifle aside for about an hour while I played with my 6.5-284.

Set up the win mag w/MS and shot 1st round 2868, hmm, shot 2nd round 2868
Crazy, then shot 3rd round wow 2868!

I stopped there to setup for distance shots but I had other obligations and had to leave.

Anyway I say all this to show, for me at least, that book COAL might be better to start with for accuracy and when you find your OAL then start charge weight.

PS I'm good with what I have now and adjust my hunting distance accordingly.
I'm almost out of time and will be only able to test this load at distance before hunting season.
Next year I might be able to improve the velocity!

This also tells me bullets set just off the lands will shoot great with best velocities.
But sometimes you just can't get there and a 300WM is a perfect example.
And also a certain powder charge will preform way different by simply changing the OAL as in velocity.

 

[Aftermath]

Over the years I have tried various methods for load development.
Here is what I do now.
I start with book value for fastest velocity powder at the minimum charge weight and work my way toward that max charge weight. I do this with the bullet just barely touching the lands. Once I see signs of pressure such as ejector mark, flattened primer, etc, I back off to the previous charge weight and shoot through the MagnetoSpeed a few times.
I don't necessarily have my final charge weight but this is the one I will use for the next step.
I start backing the bullet off the lands. In the days of yore when components were plentiful, I would load 10 rounds backing off 0.001", 10 rounds backed off 0.002", 10 at minus 0.003", etc. Nowadays, with the famine, I back off 0.005" at a time until things improve then get bad again then fine tune in between. It is truly amazing that there will be a very noticeable best seating depth. Once I find that best seating depth, I again check velocity over the MS. If that velocity is what I was (wait for it) shooting for, I'm good to go. If it is not as fast as I would prefer, I do some more charge weight testing to find where I find pressure at this new seating depth, call it depth "A". Then I experiment with seating depth again from that depth "A".
About 95% of the time, the speed is just fine and my load development is complete.