My load is 4.0 grains of TG under a cast 125 H&G boolit.

I've loaded and shot about 1000 of these before I noticed bad runout. Swapped my Lee RN seating plug for a sharp tapered plug from my Lee 9mm MAK die to rectify most of the runout along with being more careful.

How much can runout affect a handgun bullet? I'm shooting these from a shield and I'm far from the best shot. I feel that I am fretting over something that would not affect the end result unless I clamp my gun in a vice. Accuracy has been acceptable so far but I have no scientific data.

what are you talking about run out?

obvious bullet not seated straight, runout is not just for rifle calibers. Matching the seating stem to bullet profile is needed.

I've seen the same issues loading flat nose with a round nose seater. 9mm
Mine were bad enough I pulled them OP.  Not much help but I know what your saying.

Maybe contact lee for a correct plug for your bullet.

Have you tried seating your bullet in multiple steps to see if it makes a difference? I'll normally seat it part way, lower the lever if on a single or turret, rotate the shell 1/3-1/2 turn then seat. Depending on bullet length, I may do it in several steps.

I use lee, rcbs, lyman & hornady dies to reload the different pistol calibers I reload for (9mm/38spl/357/44spl/44mag/45acp). I don't worry about bullet runout in the revolver/roll crimp loads. Never saw it very more than 10/1000th's. Taper crimped semi-auto's on the other hand are a whole different animal. If my loads for a semi-auto pistol have over 5/1000ths runout, I'm doing something wrong. Why the concern??

Things like jaming the bullet into the lands and bullet setback & short start pressure come into play. Paranoid?? Perhaps. But it's better to be safe than sorry.



Most reloaders look at this picture and they see the work "accuracy" and set their loads up just like the picture above, flush with the hood of the bbl. I always set my semi-auto's up to look like the normal picture and I measure my loads targeting them to sit 20/1000th's below the hood of the bbl. That 20/100th's buys me room for bullet runout, fouling that builds up & keeps an even short start pressure which to me is more important for accuracy then where the bullet starts.

At the end of the day your concerns about bullet runout depends on how you set your loads up to start with. If you were smart and set them up so they were 10/1000th's to 20/100th's below the hood. Then you have a built in safety factor in place and your runout won't affect anything. If you set your loads up at the raggedy edge of max, then ya runout comes into play. You never said how much runout, allot of runout to me is anything over 5/1000th's.

Something to think about:
Most reloaders do the "plunk" test to make loads. They never do the "plunk" test when it's most important, namely after a range session. Especially with cast bullets!!! Junk builds up in the throat of the chambers and that buildup is measurable. I use a dummy round to measure the buildup testing the bbl at the 100round count, 200 round count & 300 round count. I also take my bullet runout into consideration. The end result is I have an idea how many rounds I can shoot before my reloading habits and fouls affect the accuracy and safety of myself and the firearm being used.

Anal??? ya!!! But at the end of the day if the reloads were setup for "max" accuracy/flush and I had 20/1000th's runout and 15/1000th's buildup in the throat of my bbl for a long range session I just greatly increased my odd of bad things happening. Add to that the use a a powder that's know for it's extremely high short start pressure, the odds just went up again. So ya, you could easily be slamming 35/1000th's of that bullet into a hole smaller than it's diameter causing pressure spikes. Or worse yet cause bullet setback.

Be safe, setup your loads so they have a built in safety margin. Have an idea of how the fouling buildup affects your oal's and keep an eye on the bullet runout.

I have found that using a large mouth flare will also help a lot in getting bullets to seat straight.

Flat bottom pistol bullets will try to tip over in the case mouth. A generous flare goes a long way in stopping this.

Motor

I think I may pursue better expanding and maybe seating dies. Definitely expansion since my die doesn't flare much.

I just switched to Lyman M dies for 9mm and 45 ACP.  I find them to be a little more gradual for an expander, overall requiring less expansion on the case.

I have a Lee Universal expanding die which is inexpensive and works well if you think that's the issue.

Expanding die:

I measure run out with a dial indicator on precision rifle rounds. For handgun rounds let the target give you feedback. Sounds like you need simple die adjustment. Throwing money at various dies will not solve the problem if they are not set correctly.

If you roll your rounds across a glass table top and see wobble then you need to worried.

The last thing you wont to do is buy a Lee factory crimp die and try to make them perfect....