Seating Depth

[Aguablanco]

Greetings,
New here and fairly new to reloading, so I have a question. While we all speak of OAL, which may be important to firearm functioning, isn't bullet seat depth even more critical? While the firearm may or may not function with a particular OAL, by seating the bullet to an OAL without regard to seating depth am I not risking over pressure? I ask this because I have been loading two different bullets for 9mm and the bullets do not have the same profile. Good news is, the shorter bullet has a shorter OAL. I have calculated the seat depth of both rounds, and determined they both seat approximately at .200 and .206 which seems reasonable. I have also gone from W-231 to Bullseye. Any thoughts will be greatly appreciated.
Thanks,
RichH

 

[Deavis]

Rich, yes the seating depth is important but that is already accounted for in the loading manual to a degree. You'll notice that shorter bullets (lighter) will sometimes have a shorter OAL but heavier (longer) bullets often have a longer OAL. However, some manuals simply show the OAL as the longest possible, minus a buffer, for the cartridge. That's where you have to make some adjustments for your gun since those data could have been taken only using a pressure barrel based and not tested for functionality in any actual gun.

If you have to seat deeper then, start at the bottom of the charge weight and work up. A quick look at pressures predicted from Quickloads shows that a difference in .010" in seating depth for 9mm with the same charge weight of W231 results in 1100psi more pressure.

 

[Aguablanco]

Deavis,
Thanks for the reply, you have confirmed my theory. Fortunately my XD-9 Tactical is not very finicky regarding OAL, it would, however, be particularly bad to have extremely high pressures. Used to reload 25 years ago on a major basis and quit when I moved to AZ. Now that I have begun shooting steel again reloading is mandatory for economy and more precise and softer loads.
While I have been using Rainer plated and Missouri BC lead, the folks here turned me on to Precision Delta and I am going to try a couple k of the FMJ. In the past I was always concerned with seat depth, and per your response, I will stay alert.
Thanks,
RichH

 

[gamestalker]

The published OAL's in our reloading books is the minimum or shortest recomended seating depth. Seating a bullet to a longer OAL/seating depth is of no issue other than feeding, and magazine fit. It's when a bullet is seated deeper than book OAL that pressures begin to rise, the deeper in the case neck, the higher the pressures will rise.

Also, OAL is the direct result of seating depth, and seating depth is what determines OAL, they are one in the same. COAL, OAL, COL or "casing over all length" are all the effect of seating depth.

 

[Aguablanco]

I totally understand, however my issue was with bullets that don't have specific data. When I was loading in the mid 80's I always calculated a seating depth as a back up number. I have been doing this currently and will certainly continue to do so. Since I have a great aversion to guns blowing up in my hands, I try to err on the side of caution. I rarely ever approach max loads, however, the 9mm is a somewhat touchy cartridge as I hear pressures can rise dramatically with only a small seat depth change. After several months, and several thousand rounds loaded, I can't believe I haven't been doing this all along. I really forgot how much fun it is and, as a bonus, I get problems to solve with the help of very kind and well informed people on this list. Thanks so much for your response.
RichH

 

[1SOW]

When I was loading in the mid 80's I always calculated a seating depth as a back up number.

I do the same now. When I can't find "specific" load data for a given bullet, I use the "seating depth" of another bullet of the same weight and type as a guide. It works, but start at the low end and work back up to your desired load. I record the seating depth of all my pistol bullets.

MY CZ and My son's XD & CZ Shadow have a little shorter chambers than most pistols, so hollowpoints, conical nose bullets and flat points in general need a slightly shorter oal than many published loads. The "push test" will give you the oal that touches the cone/lands. Subtract about .015" and that is your MAX USABLE oal in that gun with that bullet. YMMV

I also like 9mm steel challenge and our local variations too. I use USPSA "minor PF loads" with good results. 124/125 gr jacketed bullets at 1050-ish ft/min. Most of our local ranges frown on hot loads that can damage steel in a shorter period of time.

Win 231 shot well for me, but "sooty" at this speed. I haven't tried Bullseye, but have a pound of AA#5 on order to do some testing. We both use Vihtavuori n320 now--guess where my son gets it. It's the 9mm light load powder, bar none.

 

[Bmac1949]

Excellent post guys. lots of good info to work with. I'll be paying more attention to seating depth especially when reloading for the makarov.

 

[GTS Dean]

It's my feeling that the smaller the case volume, the more critical seating depth becomes. I equate it to the compression ratio in an engine cylinder. I sort all rifle bullets by base-ogive length. I weigh-sort cases based on fire-formed powder capacity, not empty weight.

On rifle cartridges, if the round chambers in a deep throat, I start with the longest overall length that will fit in the magazine, then start decreasing the cartridge ogive length by .002-.005" increments until I find the best group. On short throat chambers, I start from the lands and move back .002" at a time for best group. Bullet points, or meplats, are too variable for my purposes. Ogive length is the only reliable measurement.

 

[Aguablanco]

Seat Depth

I have noticed that a lot of guys are using N-320 and I am going to work up some loads with that as soon as I can find it locally. Funny how one reverts back to very old habits when restarting a hobby. With the set up I have, Hornady LnL AP with the Powder Cop, I have no worries about double charges. In verifying my powder drop, I have never experienced more than a .1 grain variation, which I consider insignificant at the loads I am using. I sure like Hornady's powder system much better than I did the old Lee Powder disc set up.
I am about to start a new thread regarding crimps, which I am finding confusing.
Thanks for all of the great input,
RichH