Slugged some barrels today

[wyrmzr]

I managed to have some down time today, along with lots of rain. So, I got time to slug the barrels of my S&W M&P .40 and my 300 AAC barrel.
The .40 turned out well; the measurement was .402 at the rifling, which is what my cast bullets have been dropping at. If I add PC to that, I should have a great setup. I have fired about 20 of those cast with LLA, and they fire and cycle great, but could use some more lube (a bit of leading at the end of the barrel).
The one that surprises me is the 300 AAC. It came out at .308, which isn't an issue, but.... As I pushed the slug down the barrel, at about the halfway point, I got a couple of inches where it simply slid with almost no force. So, about midway down this barrel, the diameter is larger than at both ends. I'm not certain yet, but the diameter may even be smaller at the throat than at the other end of the barrel. I'm going to slug again to confirm this, but it did give me resistance back after the couple of inches of no resistance.
I'm thinking that if the barrel is slightly larger in the middle, that may not be as much of a problem as if it's large at the muzzle. But, it does make me wonder if that will affect accuracy (I can get 1 MOA with this barrel, have fired a few hundred rounds through it). If the muzzle is OK, but the throat is really small, should I try fire lapping to ensure it's broken in, or maybe just fired a whole lot of jacketed rounds through it to break it in?
Looking for any ideas/suggestions. The barrel, for now, is temporarily out of service as I build a new upper for it; I went to .223/5.56 on this rifle by moving to a bull barrel.

[petroid]

.402 is quite a bit oversize. Are you measuring with a micrometer, or at least a good quality caliper? Even with PC you need to size to the groove diameter or .001" over. Double check your measurements, compare your caliper/micrometer to known measurements, aka factory Jword bullets. Repeat the slugging if necessary. Make sure you use dead soft lead for slugging. On the blackout, do a pound cast/throat slug. There are stickies, but basically use a dead soft lead cylinder slightly smaller than bore and drop into the throat with an empty case. Use your slugging rod to swage the lead to the throat. Eject and measure. Fit your boolit to the throat. My BLK throat is .310 and my Lee .309 push thru sizer actually puts out at .310 so the oversized sizer works to my advantage in that regard.

[wyrmzr]

I'm using a digital caliper, it is matching the measurements that others have taken.
It also matches factory jwords.
The lead should be pure, or mostly pure. I've got about 20 lbs of old plumbing lead (not solder, but plumbing lead) that I'm using for my slugs. I make the slugs simply by casting a .312 boolit out of pure lead; I'm thinking since .312 is quite a bit larger than the .308 or .309 the 300 should come out to, that's a good start.
I'll try slugging again; I do have at least one more pure lead slug that's starting at .312 diameter.

[wyrmzr]

I did retry both; same results, although the 300 AAC didn't loosen up quite so far this time. Not sure what that means yet, it basically loosened up for an inch, then had to be tapped in again, and then further down, loosened up. But it did stay the same .308, with a possible .309. The .40 is the same result.
I was thinking with the .40 slugging .402 and my boolits casting .402 I'd be OK with the .001 or .002 that PC would add. Hoping that's the case, mostly because I hate the mess that often goes with the lubes, and I know that I'd have to double up on LLA to keep it from leading.

[wyrmzr]

In that case, I won't worry about it. Initially, it felt like there was way more play than there was, and on slugging the second time, it seems that's not the case.
I'll be running a lot more jwords through that barrel once the next upper is built anyway; I think I've got 500+ 125 grain Sierras to load up yet, just waiting on the rest of the upper.

[Wayne Smith]

Did you oil your barrels before you slugged them? If not you are feeling some roughness/smoothness in the barrel most likely. I would not firelap, that is essentially simply wearing out your barrel prematurely. This barrel may benefit from a traditional break in period, however.