I recently bought a S & W, .44 Magnum Model 629-6 with a 6" barrel. Loaded up some light .44 loads( 10 grains AA#5, Sierra FPJ 220 gr., .4295 dia.) and went to the range. Shot a few rounds to sight it in but was not having any success. While some rounds would be at the center of the target many were high or left or low. I played with the windage and elevation of the sights but was having the same results. I won't say that it still doesn't need some work with sighting or that the load needs some work but I also did some reading and looking around on this and other forums for other possible causes. There was a lot of talk about checking the cylinder throat dimensions and bore diameter as a contributing cause to inaccuracy. Many people stated that if the bore diameter was larger than the cylinder throat diameter this could cause inaccuracy (makes sense). They went on to state that the cylinder throat diameters should be no more than .001 - .002 larger than the bore diameter. So I slugged the bore and couldn't get a real good reading as to the diameter. This is due to the fact, I'm told, that there is an odd number of lands and groves. I also used a Pin Gage to determine the cylinder throat dimensions and it came out to .429. I contacted Smith & Wesson and asked what their standard bore dimensions were for this model and caliber of gun. They stated that bore diameter ranged from .417 - .4182. I found that odd as the standard bullet diameter in this caliber is between .429 - .431 and if the bore is .012 smaller than throat diameter that seemed a little much. Am I misinterpreting the correlation between cylinder throat to bore information that I got??

I'd say the person you talked to gave you bad info.The throat needs to be the same or slightly larger than the bore.

There's 2 diameters of concern in a rifled barrel. The bore diameter , which is the hole drilled and reamed in the barrel blank before rifling.

Then there's the groove diameter. This is the measurement of the bore plus the depth of the rifling. Usually .010 or so larger the bore dia. Ideally , this is within .001 of bullet and throat diameters.

As you noticed , it's hard to measure a slug as S&W uses an odd-number (5) of lands and grooves. Groove dia is what is around .429-.430. Figuring an average of .004-.005 thousandths per side , the actual bore dia should be around .419.

If you have pin gages , try it.


Here's a good diagram of bore vs groove diameter in this article.
http://7.62x54r.net/MosinID/MosinSlug.htm

I suggest having someone who is known for good hand gun accuracy watch you shoot-critique your shooting, and then shoot the gun themselves. Many things can become evident in this procedure.

Thanks MKK41 that info. was helpful. That's what I'm after is the grove diameter.

Ya can actually still do it by slugging and measuring with a small-dia faced mic. Ya just get half the groove dia difference.

In addition to amazingflapjack's excellent suggestion that another shooter try the firearm, I am going to suggest that you also try a second ammunition, perhaps something commercially produced.

I would be hesitant to use a commercially loaded ammo as they tend to be more toward the upper end of the recoil scale than I want. I'm looking to get something in the 1000 - 1100 fps range. I could go to .44 Special, but would like to use the longer OAL that the gun was designed for.

Aside from being hard to find(like most ammo) , most manufacturers offer a mid-range or reduced recoil .44 mag loadings. Usually a 240gr LSWC at around 1000fps.

I will keep my eyes open and give it a try. Thanks to everyone for the help!!

Remington used to offer a midrange swc load, but l have not seen it in quite a few years. l remember reading in the past that the S&W used Federal 240 gr JHP loads for their accuracy testing of the 29/629 DX Classics.... Federal has shown itself to be more accurate than other brands in 44 MAG in my shooting

mrcrowley,

You don't have to measure anything, at least not at first. The first thing to do is slug the bore of your revolver. If the slug tightens up as it is driven through the portion of barrel which is through the frame, the barrel shank, then you have what is referred to as "thread choke", the inside diameter of the barrel is smaller through the threaded area. In the EDM barrels this is less common than when rifling was mechanically cut. If this is found to be an issue the only way to fix this situation is lapping the barrel to remove the choked area, or to "Taylor Throat" the barrel. This consists of reaming the shank portion of the barrel, removing the rifling, to the diameter of the grooves.

Once you have the slug simply try to pass it through the cylinder throats. If the throats are the same diameter as the barrel groove diameter it can be pushed through the throats with slight to moderate resistance. If the throats are slightly then the slug will pass through the throats with very slight to no resistance. The difference may only be 1-2 10,000ths". If this is what happens then everything is good. If the slg is hard to push through the cylinder throats then they are smaller than the barrel groove diameter and the only fix for this is to have the throats reamed to a diameter larger than groove diameter.

As long as the cylinder throats are smaller than the barrel groove diameter the gun may shoot moderately well with jacketed bullets, but it will never do well with cast bullets and will always be subject to severe leading, no matter what lube is used, bullet diameter or hardness!

Let me tell you a story about a 628 "Mountain Gun" I owned, It was one of the earliest EDM rifled guns. While the cylinder throats were normal size the barrel groove diameter was .435! To get this gun to shoot required reaming the throats, reaming cartridge cases to accept .435 bullets, a special diameter bullet sizing die, and lapping a bullet mould so it would throw bullets large enough to size to .435. Wasn't worth the problems so it got sold. Never heard any comments so I assume whoever bought it shot factory jacketed ammunition and never knew any better!

This was an extreme situation. I just wanted to point out that it sometimes isn't as simple as "size the bullets to fit the throats" as is often recommended. In this case the ideal solution would have been send the gun to S&W and hope they could come uo with a barrel of correct dimensions!

__________________
Gunsmithing since 1961

Jacketed bullets are more tolerant of throat and barrel diameter mismatch than lead. And you have fewer choices anyhow.

I'd try some 240 gr bullets for longer bearing surface.
And do you have some other reasonably suitable powder?

I followed ALK8944's advice and used a .429 lead slug from a Meisner Slugging kit. Drove the slug through the barrel, didn't encounter any tight spots. I numbered the cylinders and took the slug and ran it through the cylinder throats. The slug went through with only a few taps with a 4 oz. brass/nylon hammer and dowel.On the last throat the slug went through a little easier than the others. I repeated the process and found I could push the slug through with just hand pressure. I then used a Pin Gage (.429) on the throats and it slid in snugly. A .430 Pin Gage would not go in at all. So it seems like things might be o.k. in the barrel/cylinder throat arena. I would like to start working up some loads using a Lead RN/FP bullet with a .430 diameter. Any thoughts on that ?

Sir,
I had a similar situation with a 629 Classic DX I purchased. It has the 8-3/8 inch barrel, and a Burris 2x scope. I was loading a 250 grain LSWC bullet (.430") with 16 - 17.5 grains of 2400. The results were frustrating. Despite my best efforts, I could not get a group any better than 2-1/2 inches at 50 yards... and even some of those shots were fliers! -- To make a long story short, I consulted a good friend who has been shooting 44 Mag for longer than I have been alive. He told me my velocity was way too low, and that I have not found the sweet spot with that load. At his advice, I increased my charge weight to 19 grains, 19.5 grains, 20 grains and 20.5 grains. -- At 20 grains, my shot groups (almost miraculously) tightened to 3/4". At 20.5 grains, the groups opened back up to about 1-1/2 inches, so 20 grains was the ticket!

I'm certainly not an expert, but I learned a lot about the 44's sweet spot! Maybe this would give you something to try.

You will find most lead bullets are sized .431. Lead bullets are usually .001" larger than jacketed. Be not concerned. Been that way for ages and it works.