Revolver doesn’t group well

[John_R]

Three small .38s: S&W 640, Taurus 856 2”, and an 856 Defender 3”. The 3” makes 3-5” groups from a rest at 10 yards, I get 1 1/2” groups out of the others. What should I look for in the 3” to figure out why it groups worse?

If I had to keep just one, the 640 wins hands down. But the 3” with 6 rounds is a darn nice gun.

 

[Tinman357]

Finish ream the chambers, recut the forcing cone and crown. If the chambers are oversized, there isn't much you can do. Good trigger job may help.

Wish I had better advice.

 

[J-Bar]

What ammo are you feeding them?

What different loads have you tried?

 

[film495]

yea, I would try a few different types of ammo in the one that is not grouping the way you like. if you have a range rod, you can test the cylinder to barrel alignment. but, could be a lot of things. for fun, if it were me - I'd try to see how it groups loading one off rounds into the same cylinder. just cause I like to do tests and analyse data, so just me.

 

[.38 Special]

I would take a few measurements before anything else.

Begin by slugging the bore. Take a pure lead ball slightly larger than the grooves (a little undersized is fine, just smash it slightly with a hammer), grease it, and force it down the bore from the muzzle. (I like to start it with a wood/leather/plastic mallet, then push it through with a wooden dowel). Pay attention to the last bit where the barrel is screwed into the frame. It's not unusual for the bore to be a few thousandths smaller there, and that "thread choke" can ruin accuracy, especially with lead bullets. Then measure the slug to get groove diameter.

Next measure the throats. The best way to do this is with pin gauges in .001 increments. You can buy a big set to cover everything for a hundred bucks or so, or you could cover the .38/.357 bases with six or so (.354 to .359) for maybe twenty bucks. You also can slug them like you did the bore, if you happen to have more lead balls and don't mind the work.

In my experience, these measurements will usually explain any accuracy problems. If "thread choke" is the issue, the bore can be lapped - either by hand or with fire lapping - to remove the constriction. If the throats are smaller than the bore, they can be reamed to .0005 or .001 larger. If the throats are much larger than the bore - more than .003 or so - then you are pretty much hosed, though Taurus might be persuaded to exchange the gun.

There are a few other things that can explain poor accuracy, but they tend to be more rare, more difficult to diagnose, or both.

 

[PWC]

What's the difference in the grips? Fatter-skinnier, hold higher-hold lower?

 

[Pat Riot]

I had a S&W model 60 Pro 3” barrel that did not shoot accurately with ammo my models 442, 36, 10 or 19 all shot well with. It did shoot well with hotter .38 loads and was very accurate with 158 grain .357 magnum loads. The gun liked it. My wrist didn’t.

My point is your gun might do better with different loads / ammo.

 

[willb]

[QUOTE="Begin by slugging the bore."[/QUOTE]

Agreed. I would slug the bores of all three firearms and compare the bore and groove diameters of the 3" 856 against the b&g diameters of the other two. And I'm assuming you're using the same ammo in all three firearms.

 

[Archie]

Prior to getting all busy and measuring everything look at the groups. Groups should be more or less round. Ovate shaped groups indicate a shooting problem.
Wide side to side groups and a short top to bottom distance indicates you are holding the front sight more or less level with the rear sight, but allowing the front sight to drift side to side.
The up and down axis oval indicates the other way around.
Diagonal ovals show a 'yank' on the trigger and throwing the handgun in that direction. (Depends on which hand is used.) The same pattern can indicate 'heeling' or pushing the arm when pulling the trigger.

Sometimes having someone you consider to be a good shooter fire a few rounds to see if the same problems exist for him (her, it, whatever). Another factor to consider is if the trigger is as good as the other two revolvers. Perhaps a bit of trigger work (less than a full trigger job) can help this.

Once determined if the shooter is at 'fault', then move on to working on the gun.

 

[__steve__]

See if the 642 is carrying up fully all 5 positions with slow trigger pull.

Inspect for leading

inspect forcing cone

Video yourself dry firing and review muzzle in slow motion, maybe the trigger pull is effecting it

 

[CoalCrackerAl]

I cant get groups with my little H&R 32 sw long. Because of the grips being small. If i take my time and make sure im holding tight. It does ok. If i needed it for S.D.It's going to be a point and shoot from 10 ft away.