How much over bore size is acceptable?

[Robanada]

I've slugged my barrel for my makarov in at .3654", and I've cast my first batch of boolits out of my mold. Generally they're dropping in the .3678-.3682" region. Is .003" TOO oversize? Do I need to worry about unsafe pressures and size these bad boys? (Would a .366 or .367 sizing die be more appropriate?)

[rintinglen]

Probably not an issue, insofar as pressures go, but it may be that you won't be able to chamber a boolit that large once it is in a case. IIRC, the rule of thumb is that, all else being equal, you gain about 300 psi per .001 oversize, until you get up to the .004 oversize range, at which point things get dicey and the pressure rise stops being arithmetical and becomes exponential.
but that won't help if they won't feed.

[dubber123]

I would work up loads as normal, and not worry a bit. Trying different sizes may find you a sweet spot as far as accuracy is concerned, but not much else. I have a T/C Contender in 50-70 Gov't. One of my more accurate loads was a full .006" over bore size, and I had no issues at all.

[frkelly74]

If it will chamber easily you should be good to go. Size to the largest diameter that you can get to chamber easily.

[Larry Gibson]

"the rule of thumb is that, all else being equal, you gain about 300 psi per .001 oversize, until you get up to the .004 oversize range, at which point things get dicey and the pressure rise stops being arithmetical and becomes exponential."

Rintinglen

I would be interested where that "rule of thumb" comes from?

Larry Gibson

[Larry Gibson]

Robanada

Your Makarov is blow back operated. The larger the diameter of bullet over groove diameter the greater the thrust will be against the slide/breach face because the gas pushes equally in all directions. The more resistance to the bullet going forward the more the push is to the unlocked rear. oversize bullets with a given load will accelerate slide opening and slide velocity. Of course the alloy hardness and burning rate of the powder add several variables. I would suggest sizing .001 over groove diameter and work up the load as we are supposed to.

What we may get away with in locked breach firearms can become a real problem with gas operated and blow back operated firearms. We must strike a balance to make them function correctly w/o damage to the firearm.

Larry Gibson

[runfiverun]

Larry I go by that same rule of thumb.
it's been so long since I picked it up I don't remember who put it down.
but I remember seeing it and the trace.

I have seen jaxketed rounds fired going up in diameter until they wouldn't chamber and all of them fell within the standard deviation for velocity but we had no trace equipment..

[John Boy]

I would be interested where that "rule of thumb" comes from?
Ditto ... 1st I've heard about this internal ballistic 'rule of thumb' with 4 reference books in my library on ballistics

[felix]

It is always best to say nothing in nature is linear, unless defined (rationalized) by man as being such. Picking a certain load (bench gun), the round's accuracy kept creeping up when shaving necks by 0.0001 from the initial tolerance as defined by the magic marker's ink thickness. Total neck tolerance ended up being 0.0008. Changing the load, including the projectile's hardness/toughness, can and will destroy the accuracy. Everything making up the round expands in some fashion under pressure. Harder projectiles, everything else being the same, allow a closer neck tolerance which in itself increases accuracy especially with a barrel having shallow grooves. ... felix

The objective is to have DYNAMIC zero clearance between all components. ... felix

[Larry Gibson]

I just wanted to research the who and how they came up with that "rule of thumb" as I have heard it before but have not read anything concrete about it. Reason I ask is; I have conducted numerous tests varying the diameter of the bullet, both jacketed and cast. With cast in the .308W cartridge and the 30-06 cartridge 311466s of .308, .309, .310, .311, .312 and .314 were used. In the .357 magnum 358156s of .356, .357, .358, .359, .360 and .361 were used. In the .44 magnum cartridges a GB 429421 of .429, .430, .432 and .434 were used. I have yet to find any evidence of that "rule of thumb". I do measure the fps, the psi of each shot and have a trace for each tested shot with the M43 Oehler. Just interested in furthering my own knowledge as to where and how the "rule of thumb" was derived is all.

Larry Gibson

[JeffinNZ]

The 'rule of thumb' referred to is interesting. Can someone list a link please? It would surely depend on many different factors. For example, is it a lineal correlation between boolit weight? That is to say does a light boolit increase the pressure proportional to a heavier boolit? Does the alloy become a factor? What about bearing surface?

To be honest, if you are talking about 3 thou inch that's only 900 PSI and variations in components (primers, cases, powder lots) will do that anyway. I don't think the margin of safety in firearms is THAT critical that 900 PSI will result in a disaster.

[Robanada]

Awesome, thanks for all of the input, guys.

Since these are my first loads, I'm going to get a sawhorse and string, and load these as they drop, but pretty cold, and test them out in the desert. Maybe it's overkill, but I can't afford to lose any fingers.

[runfiverun]

I have been trying to remember where I first seen the 300 psi thing.
and for some reason tend to think it was in hand loader magazine back in the early 90's or late 80's [maybe sooner]
it might have been Rick Jamison, or Dick Metcalf that wrote it.

[BAGTIC]

If it fits I shoot it but after I have worked up loads with that bullet. I wouldn't just swap it in a load worked up with another bullet.

[felix]

Jeff, that 900 PSI can take a certain powder load over the top into a SEE condition, i.e., an uncontrolled (unexpected) burn rate, be it because of a secondary ignition or not. ... felix