Following the first two posts of the “The Curious Case of the Ruger Blackhawk 357 Magnum/9mm Luger Convertible Accuracy” series, a deeper look into the possible cause for accuracy variations using 9mm ammunition in the Ruger Blackhawk 357/9mm Convertible is warranted. While I cannot claim to have solved this complex issue, I found out a few pointers that may help owners maximize their gun’s accuracy with the 9mm cylinder.
The importance of the bullet geometry
In the first post, “The Curious Case of the Ruger Blackhawk 357 Magnum/9mm Luger Convertible Accuracy 1: the Accuracy Issue”, I mentioned running into surprising accuracy issues withe the Fiocchi Range Dynamics loads. This was unexpected, as this ammunition has always performed at the very least well in my guns.
Velocity and groupings table for the Ruger Blackhawk 357/9mm Convertible and Stainless 357 Magnum. Groups shot rested on sandbags at 25 yards. Velocities measured with a Caldwell Ballistics Pro Chronograph placed 10 ft from the muzzle.
When I prepared to re-run the accuracy test for that post, I made a bevy of measurements on the two different lots for comparison and in case they would yield valuable information. It turns out this helped figuring out what bullet parameters help maximizing the Blackhawk’s accuracy in 9mm.
Twenty cartridges of each Range Dynamics 115gr FMJ lots were randomly selected and numbered. The bullet diameter, case mouth diameter, case length, overall length (OAL) and weight were measured on these 2×20 rounds (summarized in table below)
The lot made in Turkey shows higher variations in all measurements but the overall cartridge length than the lot made in the US. Counterintuitively (at least to me), the former grouped better than the latter despite its manufacturing being less consistent (5.11’’ vs. 7.52’’). Also, the accuracy problem is confined to the Ruger Blackhawk Convertible: both these lots of ammunition perform at least adequately if not very well in the other platforms I regularly shoot them in.
To find out if some obvious construction or geometrical differences between the bullets could play a role in these noticeable accuracy differences, I pulled one bullet from each lot and the Winchester 9mm NATO FMJ-FP Target, the most accurate 9mm load in my Convrtible, for some visual observation and measurements (table below).
The Winchester 9mm NATO load has several distinctive characteristics not shared with the other two loads. Its bearing surface is the longest, its base is deeply hollow and uniform, the nose is flat, and it is loaded at higher pressure than the average 9mm Luger. One revealing point is that compared to the Fiocchi loads, the Winchester 9mm NATO sees an increase in its velocity compared to the factory number, whereas the other two do not.
This points toward a better and consistent obturation than for the other two bullets. The bullet construction of the Winchester load is conducive to good obturation: its base has a pronounced and uniform hollow cavity that will help the skirt be pushed against the throat by the expanding gases (especially since this ammunition is loaded at higher pressure than the average 9mm Luger), providing a better seal and more consistent pressure curve. This will also help the bullet center itself in an oversized throat and enter the forcing cone with proper alignment.
Oversized throats as the accuracy robbers
Though well behind the Winchester load, the Fiocchi 115gr. brass FMJ bullet showed better groups than the more exact Fiocchi 115gr. copper jacketed bullet. The former has a hollow base, while the latter does not. While its base is significantly shallower than the Winchester 115gr. FMJFP, it likely helps making it more accurate than the flat based 115gr. copper jacketed bullet, despite looser manufacturing tolerances.
If some bullets obturate better than others, it means that there is room on the sides of the throats and that these other bullets cannot consistently fill that space upon firing. To confirm this hypothesis, I compared the throat diameters of the two cylinders.
I picked a Hornady 158 XTP bullet measured at an exact 0.357’’ diameter and an Hornady 147 gr XTP measured at an exact 0.355’’ diameter. I was able to push the 158 gr XTP snugly through the six throats of the 357 Magnum cylinder with the same gentle pressure from my fingers with the bullet properly aligned, indicating consistent and adequate throat diameter of 0.357’’.
Even though I expected it by then, I was disappointed to see the 0.355’’ 147 gr bullet fall through each of the throats of the 9mm cylinder. The larger 0.357’’ 158 gr bullet only required good alignment and gentle pressure to go through each throat. I had found the accuracy robber: oversized, 0.357” throats in the 9mm cylinder.
Both of the cylinders’ throats were likely drilled on the same set up and with the same diameter tools. This works well for the wider 0.357’’ bullets in the shorter throats of the 357 Magnum cylinder, but degrades accuracy in the long throats of the 9mm cylinder.
Bullets that cannot deform enough to obturate in the throat will not consistently seal the gases and pressures will vary from shot to shot. Furthermore, these narrower bullets may enter the forcing cone misaligned or perhaps even inconsistently deform upon ramming the forcing cone, deteriorating accuracy. In my Ruger Convertible and its 0.357’’ throated 9mm cylinder, only cartridges with wider bullets and/or consistently obturating stand a chance to be accurate.
Conclusions
People experiencing poor accuracy with their 9mm cylinder have not been inventing this. In my Ruger Blackhawk Convertible, the accuracy of the loads ran in the 9mm Luger cylinder and its oversized throats clearly depends on their ability to obturate and seal the gases in.
A sample of one can hardly be used to extract general conclusions on any matter, but I suspect that my gun is not alone in the fact that both its cylinders are machined with throats of the wider 0.357’’ diameter. This, sadly, can decrease the appeal of these guns. If their construction makes them especially sensitive to the quality of ammunition and requires tailored handloads or higher end (thus more expensive) 9mm Luger ammunition to do better than just plinking, this may not fully justify investing in a convertible model.
For those experiencing subpar accuracy with their 9mm cylinder, trying out different ammunition may bring good results. Loads known to feature concave bases, longer bullet and/or higher pressure will likely be conducive to increased accuracy.
The Ruger Blackhawk Convertible is a two-faced gun capable of stellar accuracy when using the right ammunition, and a honest plinker with the rest. As always, things come in shades of gray rather than black or white. At the end, all the people on the forums discussing the accuracy of this gun were right in their own way. Their experience was likely tied to the their luck (or lack thereof) of finding a type of ammunition that agrees with their gun.