Questions about OAL

[ron59]

You know me... 30,000+ rounds of 9mm in less than two years. I certainly understand the relationship of OAL to powder, have somewhat of a deeper question.

Those 30,000 rounds were pretty much all Montana Gold 147grainers. I've *just* switched to Precision Delta 147grainers. Now... the PDs are longer (.637" for MG, .663" for PD). I was pretty happy with my previous load, so I decided to try to keep the same amount of bullet inside the case, and change my OAL from the previous 1.120" to 1.145" or so. This "worked" fairly well, as when I chrono'ed this round, it was only about 6fps faster than my previous load with the MG bullet.

Now... a buddy of mine who's also on this forum, is loading his PDs at 1.125" or so.

What's the difference? By that I mean... is there a rule of thumb about how much bullet you want inside the case? The more the better? The less the better? Or is this totally a factor of "whatever works best in your gun type of thing"?

I'm trying to understand the science behind how much bullet inside the case. I realize that running the shorter OAL, you'd need less powder for an equivalent velocity... just wondering if there might be a side effect of increased/decreased accuracy by going either direction or if it's a total toss-up.

[fredj338]

As you know, deeper seating in handgun rounds produces more pressures & often higher vel. As to accuracy, I doubt any shooter can tell the diff. So there is no rule of thumb, as diff powder/bullet combos will react diff, but the only rule I know is seating deeper WILL increase pressures.
Now rifles are a bit diff. It's a good rule of thumb to keep at least one caliber length into the case mouth for rounds that have to feed form a mag. Less than that & the bullet can be "cocked" as it hits the feed ramp. You lose neck tension & accuracy, so for rpeating rifles, bolt or semiauto, I stick to the one caliber minimum. On my single shots, you can go as skinny as you like, but the one caliber rule works for me.