Loaded some 9mm 147g Hornady bullets with 4.2g of Blue Dot. (lowest starting load listed)
Used Federal primers
Brass trimmed tol 0.749" +/- 0.002"
COAL to 1.100" +/-0.003" straight from the manual.

Primers seemed to seat very easily when I put them in. This was once fired brass from Walmart WWB.

Shooting them they have very little recoil. 2 of 20 actually failed to cycle the slide (Sig p228) fully.

Primers appear slightly flattened compared to factory WWB fired from same gun.

I doubt there is a pressure issue, but have no explanation for the flattened primers...

Is there any other issue I'm not thinking about???

Look at the primers on your loaded handloads... Could you be crushing the primers as you're seating?

Federals are soft primers, that could be part of it.  FWIW  I've also noticed some flattening of primers on 9mm loads that I know are well below max.

They do not look crushed before firing....hmmm.

Maybe I should try CCI primers and see if same thing happens?

I think your problem is caused by the trimmed cases.  I believe when the trigger is pulled, the firing pin is pushing the case forward against the chamber shoulder before the primer fires.  When the primer ignites the powder, the pressure in the case rises pushing the primer reward while the case is still against the headspace shoulder.  As the pressure builds the case moves rearward smashing the primer against the breech face thereby flattening the primer.  The primer is then seated flush with the base of the case when the case hits the breech face during max pressure.   I have to question why you are trimming your cases?  Were they too long to chamber or over max case length?  I have loaded thousands upon thousands of 9mm and have never had to trim a 9mm case.  I measured several thousand cases that had been fired numerous times and have never seen a case that was over max case length.  However I have seen flattened primers using a normal load of 231 with 124 gr bullets.  When I started looking for the cause, I found the flattened primers all occurred on cases that were .003 to .005 shorter than the other cases that didn't exhibit the problem.  That being said I do not use Blue Dot in any pistol calibers as I feel BD can be very temperamental when used in small pistol cases.  I would recheck my load data to make sure you have the right data, then proceed with caution.

G

Might be a good idea... The other guy hit it on the head when he said Federal's are REALLY soft primers.  I won't even use them for that reason.  I go for CCI or Wolf only.

Thank you.
I will try a different powder. I had some concerns about the blue dot since it is described as a magnum powder, but was listed in the Hornady manual as an acceptable choice. I cross-checked the burn rate and it seemed to be middle of the road.

I do hope it's a powder issue and not the Fedral primers, as I have one pound of the blue dot, but like a lifetime supply of the primers!

Any recommendations on powder?

I was afraid that might be the case. .   There are no other signs of overpressure.

If the Federal primers are indeed that soft, how does anyone use them without getting flattened primers?
Also seems to make being able to detect actual overpressure issues that much more difficult.

I hate to change both the primer and powder the same time, since I won't know which is responsible, but I'm concerned about using the Blue Dot now, and I don't want to take any chances with blowing up the gun. Oh well, better safe than sorry.

Thanks for all quick replies so far BTW!

I'm also going to skip trimming cases unless over max length.......

I have not had any problems with Federal primers in 9mm and .357 Mag,  I don't use them very often because they are a lot more expensive here than any other brand.  I also do not like the huge boxes they are packed in.  I was just looking at my Lyman 49th Manual and they list 4.6 gr of Blue Dot as the starting load.  Your OAL seems a little short, is the length listed for this bullet in your manual?  The 9mm is very sensitive to OAL and pressures can spike very high if a round is loaded short.  Have you chambered a couple of rounds and then extracted them without firing to see if the bullets have been pushed further into the case  when they were chambered.  Have you removed the barrel from your pistol to check to see if the rounds drop easily into the chamber.  Then check to see how far and easily the WWB rounds drop in.  I would try loading a few in untrimmed cases to see if the short cases are part of the problem.   I use Power Pistol and Unique for 147 gr  loads in 9mm and have no problems.  

G

I had extreme pressures, flattened primers and high velocity using starting loads of Blue Dot in .41 Magnum. Grossly different cartridge, but the same general problem. They have since come out and warned against the use of Blue Dot ever in .41 Magnum. Obviously other people have had issues too.

I never trim straightwalled handgun brass. It just doesn't need it. Accuracy is great and most pistol rounds headspace off the case mouth. Shortening the case length increases headspace as the previous posters have mentioned. It allows room for the primer to back out.

Blue Dot is at home in 9mm when loaded hot and using heavier bullets. 124's and 147's with close to maximum charge weights. I would dump the trimmed 9mm cases, or seat the bullets long enough to just touch the rifling. This is a way to stop the case from moving too far forward in the chamber. Your measurements must be precise and your seating practices monitored closely. Too long and the bolt won't fully close while danger lays and waits.

I really like Power Pistol for 9mm and .40 S&W.

Try WSF. Federal primers along with the box they are packaged in are the bottom of my favorite list for pistol loads.