# Bullet seating depth question



## Bird (May 10, 2005)

I'm working up some loads for my 300 Blk. I'm using the same powder charge, case, primer and bullet weight (220 grain). 1 set is with hollow point boat tail and the other is with round nose flat base bullets. Obviously the HPBT's are longer than the RN's. I set the seating die up for the HPBT's first and then loaded the RN's. The RN's' are about 0.080" shorter than the HPBT's measured with a comparator tool from the ogive to base. COAL for the loads are 2.260" HPBT and 2.100" RN.

My question is should I load to the same seating depth by the die or adjust bullet seating depth to ensure same air space inside cartridge? My thoughts being additional air space inside the cartridge will have different pressures and not be an even comparison of the rounds. Am I over thinking this? Should I really just worry about how the rounds print on the paper?


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## mrsh978 (Apr 24, 2006)

Use max aol- and adjust for your guns accuracy - just make sure bullet is not too long and sitting in lands once round is chambered


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## Ernest (May 21, 2004)

Air space in the brass, assuming nothing weird, is not really a big variable here. Seat the bullets to recommended COAL in the reloading book. From there, you can begin making tweaks.

If the rounds are jammed into the lands, on the AR platform, many times the loaded round will not eject easily. So, just load one of the round nosed bullets in your weapon, and see if you can eject it without butt bouncing the weapon.

Better approach would be to pull the ejector from the bolt and reassemble the BCG completely except for the ejector. Then, you can load a dummy round with the bullet seated real long. With light finger pressure, try to close the bolt. It will likely refuse to go into battery. So, seat the bullet a bit deeper, and try again. With trial and error you will quickly find the max. COAL for that bullet in your weapon. When the bullet is just short of the lands, the bolt will close completely and open easily with just light finger pressure on the cutout.

Don't use the charging handle or allow the bolt to slam home with spring pressure for this technique to work.

If you try this technique with the ejector in place, it tends to give false measurements.

The round nosed bullet and the HPBT will have different pressures if loaded to the same COAL and with the same powder. Thats because they are different bullets. Now, the pressure difference might be real small, or it could be relatively large. If the two bullets were made of the exact same material, weighed the same, and had the exact same exterior diameter, there would be a pressure difference due to the difference in bearing surface.


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## BradV (Jun 27, 2012)

In regards to it being a .300 BLK round, and I have seen previously that you shoot through an AR, I would adjust the OAL of that round nose to give the highest reliability in your AR. The round noses can be very picky on if they will feed or not. 

You are correct in that the different amount of unused case capacity will cause a pressure difference. My answer would be again, adjust the seating depth of those rounds for function, and adjust your powder charges as necessary to keep them subsonic. Fine tuning for accuracy would be after finding what is going to function properly in the gun.

If you plan to single feed them or are shooting out of a T/C or Handi Rifle, I would load several batches and go through some trial and error to find the best seating depth and powder charge combo for each particular bullet.

The 220 SMK loaded to 2.12" should feed well from an AR-15.


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## THE JAMMER (Aug 1, 2005)

Totally agree with Ernest. The amount of air in the case will have a minimal effect on accuracy when compared the major effect the distance the bullet is off the lands. sometimes as little as 5-10 thousands will make a major difference in accuracy.

As was stated before: start with max COAL, and work from there. Some guns will allow a cartridge to fit at more than sammi coal. Just be careful you are not engraving the lands, as was said before.

Good luck,


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## Bird (May 10, 2005)

Thanks guys. FYI, it is a High Standard HSA-15 M4, subsonic loads and shot suppressed. I loaded the 220g SMK's to COAL per Hodgdon load sheet, 2.260", based on AR, 16", 1:8 twist, carbine length gas tube which is pretty much what I have. It incidentally, it the max I can realistically load and still fit the magazine. I'll load up another batch of the 220g RN's to a longer length and compare chrony reading. Going to the range Monday anyway so I'll just shoot more :biggrin:


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## Bird (May 10, 2005)

Ok, finally got to the range with the chrony today to finally test the SMK's and the RN's. I loaded both with 11.3 and 11.5 grains of H4198. As mentioned, the SMK's were 2.260 coal and the RN's were 2.100 coal which, is about .10" longer than seating depth of the SMK's.

There wasn't much difference between the 11.3 and 11.5 grain loads, for their respective bullets (20 fps avg) but there was 100 fps average difference between the SMK's and the RN's with both powder charges. The SMK's were all super sonic and the RN's were all sub sonic.

No problem feeding, cycling, or holding bolt open with RN's or SMK's. All loads shot suppressed and grouped well. Back to the loading bench...and that's the fun part :biggrin:


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## Griffin (Aug 2, 2006)

Glad to hear about the improvement, sounds like the powder change made a difference, especially with cycling.


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## BradV (Jun 27, 2012)

How was the extreme spread on the velocity? I am wondering if I could be looking into a powder other than A1680 for function that gives a bit more consistent shot to shot velocity.


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## Bird (May 10, 2005)

BradV said:


> How was the extreme spread on the velocity? I am wondering if I could be looking into a powder other than A1680 for function that gives a bit more consistent shot to shot velocity.


Not good enough yet to send me scrambling to get more. I haven't crossed H4198 off the list yet as I've got to work a few more loads. I am pretty careful about case prep, load consistency, and seating depth so I was surprised to see as so much spread in velocity although the groups were good.

220 SMK (11.3/11.5) ES: 49.8/73.2 
220 RN (11.3/11.5) ES: 74.7/135.8 hwell: That sucks but I've got some changes to try to get more consistent. The SMK data wasn't too bad and it indicates that slightly less powder might be the ticket since all SMK loads were just barely supersonic.


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## BradV (Jun 27, 2012)

What I normally see with A1680 is that the higher the powder charge, the lower the spread. I am assuming it is simply a better burn being a higher pressure load. The trick is only finding the sweet spot where I can get a good ES while remaining subsonic. I have some magnum primers to experiment with but havent found the time. Some people seem to report improvements and others claim no difference. The same goes for annealing and crimping. I crimp everything with a Lee factory crimp anyhow.


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