Barrel slugs not consistant

[bayjoe]

Starting to work on casting bullets for my Marlin 30AS. It is a Micro Grove barrel. What is odd is when I slugged the barrel. It miked .3081 just past the chamber and .3094 at the crown and midway down the barrel. The slug started like normal then halfway down the barrel it got loose. At first I thought it might be fouling, so I cleaned the barrel, even used an Outers Foul out system on it, to make sure there was no copper fouling. I slugged the entire barrel twice, slugged just past chamber, maybe 2 to 3 inches, twice. And then slugged from the crown to half way down the barrel twice.
My question is:Has anybody seen a barrel mike like this? I am assuming to set my bullet diameter at least .001 past the widest measurement in the barrel?

[TXGunNut]

I've noticed tight spots near sight dovetails on a couple of 336's. A .311 boolit will likely do just fine.

[btroj]

I would go .311 and see what it does. If it leads then fire lap to remove tight spots near breech. Reverse taper like that can cause leading in some cases.

In the end, shooting is the way to really know.

[runfiverun]

i'd mail it back to remington with a note explaining how a barell should be made.
i once mailed an entire shotgun back to franchi in a flat rate box with a nice note.
so it could just be my attitude.

[longbow]

Yeah, I read about tight spots on the Marlin owner's forum and since my .44 mag was leading I decided I would slug it. Sure enough, tight spots under dovetails. Apparently the roll stamping is also a culprit, and maybe so but I didn't notice that.

I decided to hand lap mine even though it is microgroove. I wanted to work on the tight spots only and figured fire lapping might not be so specific (never tried it though so speculation on my part). Anyway, I hand lapped the tight spots then did a progressive lapping from breech to muzzle using fewer and fewer strokes near the muzzle to try to put a very slight taper in the bore.

Now it is much more accurate and leading is a thing of the past.

I was worried that with microgroove rifling, lapping might wipe out the rifling but it turned out well and wasn't difficult to do.

My vote would go to hand lapping if you plan to lap it. Fire lapping may work well for you too especially since the tight spot is near the breech. Either way, with microgroove rifling I would use a fairly fine abrasive. I used valve lapping compound of around 400/450 grit and that turned out well. Finer might be even better.

Before lapping, you might just try a larger boolit to see if that works. At around 0.001" squeeze at the tight spot I doubt it will fix the problem though.

Just my $0.02 worth.

Longbow

[popper]

My 336 is tight at both ends and loose in the middle but I'm waiting to see if PC solves the problem before lapping. I figure tight at chamber(roll stamp) swages down, then you get gas cuting in the middle that gets 'run over' under the dovetail, with leading.