I have 1980 M25-5 with an 8 3/8" bbl. Have read that the oversized cylinder throats give poor accuracy with .451 bullets. This makes sense but barrel length should have an affect on accuracy also. It seems like that a bullet "wobbling"down an oversized throat into the forcing cone would be pretty much stabilized by the time it reaches the end of the barrel, especially in an 8 3/8" bbl.
Are my thoughts valid is there other factors involved?

The issue really comes into play with cast bullets. The bullets wont seal up properly in an oversized throat which lets gas by and causes leading.


If you shoot jacketed bullets you probably wont notice anything.

I agree with Curtis.

In 2014 I had all of my Model 25 throats measured by an engineer using pin gauges. All of the cylinder throats for this Model 25-2 measured at .455.

I always use .45 ACP FMJ ammunition at the range. The target accuracy results in the attached file were achieved at 25', which is my personal defense engagement distance standard.

I am very pleased with the results.

Attached Images

Smith & Wesson Model 25-2.jpg (37.9 KB, 63 views)

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I have a 25-5 with 8-3/8" bbl just like the OP's, and it patterned like a shotgun, as in 8" groups from a rest at 25 yds. using 255 gr LSWC. It was the worst shooting Smith I've ever owned. Throats measured .457 measured with telescoping and small hole gauges.
Many people on this forum and elsewhere have reported satisfactory groups with .455 and bigger throats using .454 lead bullets or jacketed bullets. I don't want the expense of using jacketed nor do I want an odd sized stash of bullets just for this gun.
I had the Smith PC install a new cylinder, but a pinched nerve has prevented testing the new setup.

Why would a jacketed bullet group better than the same size cast lead bullet in an oversized chamber as long as the barrel is properly sized?

Because the copper jacket won't gas cut or partially melt from blowby combustion gas like lead will. The hot combustion gas can soften and even gas cut the lead, leading to severe leading of the barrel. At least that's how I understand it.

The first thing I would suggest is that you get some pin gauges and measure the cylinder bores. If your 25-5 is a non pinned barrel, it may not have oversized bores in the cylinder. The barrel length made no difference in my 8 3/8 25-5 when I used 45 ACP ammo in it-it still key holed badly. I load, and all I use is lead. The fix for me after I had the gun's cylinder cut for moon clips was to use 45 Winchester Magnum brass-which is like ACP, but much longer. I loaded to 45 Colt levels, and it has become a favorite gun. My non pinned gun's cylinder was the right size.

My suspicion on accuracy problems with oversized throats.

The better everything is centered the more accuracy.
Not only is it important that the bullet be centered from firing to its transition to bore but, the bullet need to stay on ITS CENTER during firing and flight.

I think that if a bullet enters the forcing cone on its true center, but, a bit off center to the forcing cone, it gets centered to the bore and continues on its way fairly well as is corrected in the forcing cone and initial part of the barrel. Shouldn't mater long or short. That is what the forcing cone is for to help align everything as the bullet goes from cylinder to bore. If the cylinder bore alignment was always dead perfect there would be no need for a forcing cone.

But, if the bullet leaves the chamber with some yaw (the axis of the bullet is of center) it may get centered in the bore but be knocked out of balance on its center and after it continues down the bore and then through its flight path it spirals outward. There is a huge pressure release on the base of the bullet on firing and good throats help keep the bullet aligned true on the bullets own center as it leaves the case, oversized throats do not and allow the bullet to be cocked a bit off its own center.

I think the problem with oversized throats and chambers is that the bullet is far more apt to get cocked a bit on its axis upon firing than it would a good chamber and end up spinning farther off its true center than if it got to the forcing cone true on it center.

Bullet slightly out of bore alignment but on its centers as it strikes forcing cone causes small degree of accuracy loss.
Bullet off its center as it strikes the forcing cone causes a far greater degree of deformation and accuracy loss

If you have a perfect bore, and chamber in a match grade rifle, poor grade bullets without uniform jacket thickness or cases without uniform neck thickness will show pattern dispersion too because the bullet doesn't spin on its center once it leaves the bore.

As it's been stated above the problem with oversized throats applies to cast bullets. Through much trial and error I've found that Bullets .001 larger than the throats size are best for accuracy. A great solution for oversized throats is to load plated bullets. Rainier or Berry's work well for me. If you search online John Taffin penned a great article on this years back.

A bullet doesn't even make one rotation inside the barrel. To expect it to stabilize is wishful thinking.

Like others have said jacketed bullets don't see as much of an effect, but lead bullets can be problematic with oversized throats.

Back when S&W reintroduced the Model 24, .44 Special, I wanted one, as I had read the originals were extremely accurate. The day I bought one, a blue, 6" barrel, I also bought factory .44 Special ammo.

The next day I mounted it into my Ransom Rest and was very disappointed how it shot. The best I could get with the factory ammo was an average of 4 1/2" at 50 yards and the worst brand was 18" (eighteen inches) with name brand ammo.

Being very disappointed, I measured the cylinder throats and they came out being .434" diameter and the 5 groove barrel came out to .4295".

I had intended to use cast bullets in this gun anyway, so cast bullets and I always used linotype as the casting material and they came out at .433" diameter. I didn't size the bullets but only lubed them.

Back to my range and used the Ransom Rest again and I think I used Unique powder with different charges, I finally was able to get groups that averaged 2 3/4" at 50 yards, however, after about 25 shots, I had to brushed out the bore as it was shooting an oversize bullet down the bore, as group size would start increasing again. The worst part was getting the residue out of the cylinders, right where the case mouth rested to the starting of the throat and it was very hard to remove. It had to be brushed out, as bore cleaner wouldn't touch it. I had pretty good results with the Lewis Lead Remover Tool as it used brass mesh to remove the lead and hard residue.

After about 2 months of this, sold it, just too much trouble.