Keyholes in targets ???

[Blackhawker]

I checked roughly the first 20 pages of the thread for past questions referring to this topic and found none so I figured it was appropriate to inquire.

Exactly what causes excessive keyholes in targets?  I realize there are probably 1000 combinations of things that can cause such problems so I suppose I should narrow them down with specifics on a given load.

I have a W&H Buffalo Classic with an additional 38-55 barrel.  I've been loading 265g FNGC's in it.  I have used IMR 3031, IMR 4198, Unique, and 2400 in my loads.  All have shown to be quite accurate considering I'm not much of an iron sight shooter but most holes touching at 50 yards I figure is pretty good for me.  Basically I've found with the 3031 and 4198 that if I jack up the charge too much, my groups get larger so I have narrowed down my loads to the best charge vs groupings for each powder as typical handloaders do.  Unique shoots great but the published charges give somewhat low velocities and I am looking for something a little "hotter" than those. 

Here is where my question begins:  I've decided to "play around" with 2400 recently.  I've found that there isn't too much info published for 2400 with the 38-55 but I found an article listing a given charge that seemed to be fairly accurate. 
LINK: http://www.brimstonepistoleros.com/gazette/jan06.html
Indeed, I found the article's listed load to be quite accurate but I, in search of a little more velocity, have decided to "play around" with the charge despite the fact that I have no other data to go on.  Basically I've increased the charge as far as a light to mid load for a 375 Winchester as listed in the Lyman 49th edition.  I felt this was an OK (safe ) thing to do as the W&H BC frame will handle the pressure and since the 375 Winchester is a shorter cartridge, it would develop higher pressures than the longer 38-55.  I also compared similar data to other powders as well but that's a little "iffy" since powders all have different burn rates.  (OK, I might have been going out on a limb and getting a little risky)

Well, to make a long story short;  I haven't had any bad things (dangerous things) happen with the load other than horrible groups and lots of keyholes.  I'm going to scrap the higher 2400 loads since they've proven to be terribly inaccurate but I was wondering if anyone has any insight on what may have cause such keyholing (sp?).  I mean these were BAD!!  At 75 yards they were flying anywhere within a 2ft to 3ft diameter and I'd say 30 to 40% were keyholed.  I suppose this has something to do with the 2400 powder and it's burn rate since I saw nothing similar to the this behavior with the IMR powders.  Can anyone explain, roughly, the details or physics of what's going on here.

[Grumulkin]

Keyholeing is caused by lack of bullet stability which in turn is caused either (usually) by a too low twist for the length of bullet or insufficient velocity to spin the bullet fast enough.  To hot a load will cause pretty big groups but I've never had this cause keyholeing; I won't say it can't happen though.

[Mikey]

The article showed those bullets sized up to .381 to .383 and if the 265 gn fngc slugs you are shooting do not match the bore diameter you will have keyholes and lousy groups.  From my older Lyman manual, 43rd edition, I see loads in the 38-45 with 4198, 3031 and 2400 with a jacketed bullet but nothing listed for accuracy for a cast slug.  This manual mentions groove diameters larger than .377, yet others on this site shooting the 38-55 have stated their rifles prefer cast slugs up into the .380 diameter range. 

I once had the same problem with a 444 Marlin - I tried shooting 375 gn slugs from a 43 Spanish that had been sized down considerably to a .430 diameter.  Over a charge of 3031, which should have been ok for the cartridge (not over pressure), all I got was keyholes.  I did not find out until later that some 444s would only sghoot cast slugs in the .430-.432 diameter range.

You did not mention the diameter of the 265 gn slugs you were shooting - if they are small for the bore youwill get lousy accuracy and keyholing - betcha. 

[Graybeard]

There have in the past been quite a few discussions on the topic and in fact there is an active one ongoing now I think on the NEF/H&R Centerfire Rifle Forum.

In by experience it is most often caused by one of perhaps three things all eventually going back to failure to properly stabilize the bullet. If velocity is too low which is what I think is the issue in the active thread I mentioned the bullets will tumble. If the bullet is too long for twist rate it will tumble and if the rifling in the barrel is defective it will tumble which was the case with a brand new Taurus M44 revolver I bought once.

Ultimately it all boils down to lack of stabilization you just have to figure out why it failed to stablize. If the bullet diameter is too small for bore size that will prevent it from spinning enough to stabilize.

[Blackhawker]

Thanks for all of your replies.  You all gave me one of those knocks in the head while I said, "I could have had a V-8!)  DUUHHH!!     All of your comments make sense and all point to lack of stability due to too low of twist rate.  I guess what had me puzzled was that while I jacked up the powder charge, one would expect to increase velocity and spin and therefore maintain the stability.  However, as Mikey brought up, I did not indicate bullet diameter, and as Greybeard (Bill, William is your name, correct???...which do you prefer??) said; If the bullet diameter is too small for bore size that will prevent it from spinning enough to stabilize. I believe he has hit the nail on the head. 

OK, this is gonna seem a little odd here but I actually DO realize that typically one chooses a cast bullet .001 inch over the bore diameter, I just didn't do it here in this case and my crucial mistake is perfectly clear now.
Although I've got some good accuracy out of the rifle thus far, perhaps the heavier charge of 2400 (a fast burner) has pushed things beyond the limit of the bullet vs. bore diameter.  My barrel slugged out just a little over .379, seeming to be about .3795.  The bullets I use are .379.  I chose them when first loading for this rifle because I had heard some of the .380 bullets would bulge the neck of the case enough that it would not allow for chambering in some of the under-chambered H&R rifles.  The .379 fit well and proved to shoot well on my first trip to the range.  Once zeroed in, the very first four bullets made one elongated hole at 50 yards....and this was the first time I'd ever fired a 38-55 to boot.  I was impressed and have been impressed since the rifle has shot well all of the time thus far so I never thought twice about the .379's since they're shooting well (at low to mid velocities and pressures).  It appears that maybe pushing the pressure with a hot burning powder is not allowing the slightly smaller bullets to engage the rifling properly and therefore they're losing twist and overall stability. 

Mikey, I have that same old edition of the Lyman manual as well as the new 49th edition.  In the newer version, they have loads listed for the above mentioned IMR powders for cast bullets as well but still none for the 2400.  The loads are definitely heavier than listed in the older version, perhaps to accommodate the newer and stronger 38-55's. 

Thanks you guys!  I guess I've gotta buy some .380 or .381's and hope they chamber and then go from there.