Velocity-To-Charge Ratio Revisited
January 04, 2011
Lane used the V/C ratio method and Ruger Model 77 Hawkeye rifles to develop handloads for the .300 RCM and .338 RCM when no published data existed.
In last October's issue, I described an empirical method one could use when working up a new handload. Apparently I didn't do a good job explaining how to apply it because I received several inquiries from interested but puzzled readers. Let me make amends by emphasizing the key aspects of this handy scheme:1.) You must chronograph your handload's average velocity.2.) You calculate the V/C ratio by dividing the average velocity of a batch of test loads (at least 10 rounds) by the propellant charge weight. For example, .30-06 cartridges charged with 57.3 grains of Reloder 19 should deliver approximately 2,700 fps in a rifle with a 24-inch barrel (according to Sierra's 5th edition reloading manual). The V/C ratio is 2700 Ã· 57.3 = 47.1.3.) Dynamic V/C ratio is the increase in velocity resulting from each grain the propellant charge is increased. For example, if you step up the powder charge by 1/2 grain and the average velocity increases 32 fps, then the dynamic V/C is calculated as 32 Ã· 0.5 = 64.4.) As you work up your hand-load, you should compare the dynamic and average V/C values for each increase in charge weight.
Using the values from 2.) and 3.) above, the ratio of the dynamic to average V/C is 64/47.1, or 1.36. If the dynamic value exceeds 1.5 times the average V/C ratio, you must repeat the specific test load to verify the value. If so, the data suggests your handload is surely approaching the maximum limit, so you may choose to proceed cautiously if there are no other indications of excessive pressure.5.) If the average V/C ratio is lower than the value calculated using published load data, your handload/rifle system is less efficient than the one used to develop the load manual's data, so you may have to exceed the recommended maximum charge weight to achieve the corresponding maximum velocity. Do so carefully in small steps while watching for obvious pressure signs. Conversely, if your loads exhibit a greater V/C value than the published data, your system is apparently more efficient, and you may reach the maximum velocity listed using less than the corresponding maximum charge weight shown in the manual. Never try to exceed the maximum velocity. The laws of physics regarding energy conversion are quite rigid. If your bullets chronograph even 50 to 100 fps faster, then you're surely exceeding maximum safe working pressure.
Typically, for many reasons, a handloader gets an itch to develop a load for a favorite rifle using a new propellant he hasn't tried before. I started out loading for my .270 Winchester with Hodgdon's surplus 4831. At only a dollar a pound, I didn't mind the dealer scooping it out of a 50-pound keg and pouring it into a small paper sack. Later, I discovered IMR-4350 might be a suitable option. It was twice the price of 4831 but came in a sturdy can that stored on the bench better, plus I thought it might launch the bullets faster and maybe even straighter. I checked out DuPont's load-data sheets and made up some test loads. An electrical-technician friend had recently designed and fabricated a homemade chronograph for me. It was crude and cumbersome to use, but it measured bullet velocities, and as an engineer, I could always use more data.
Looking back at my records and comparing them to the old DuPont recipes, the average V/C for my Winchester Model 70 was 52, whereas the V/C for the DuPont data was 54. As the powder charge increased, the average V/C remained almost constant, and the dynamic value was stable, i.e., increasing essentially linearly without abrupt change. However, according to the V/C theory, because the ratio for my handloads was lower than the DuPont V/C ratio, my handload/rifle system was somewhat less efficient.
In hindsight, I could have safely bumped those early handloads to even greater velocities. I would have had to load more powder than the maximum IMR-4350 charge weight recommended by DuPont, but the groups were near MOA, and another 100 fps wasn't that attractive considering I'd surely be eroding safety margins.
Of course, with the plethora of new cartridges and propellants introduced during the last decade, the situation might arise when you want to develop a handload for a new round with a well-known propellant or even a new one before any published data is available. In fact, I've recently tested handloads for two new rifle cartridges with both conditions. I have enclosed a set of charts that gives the details.
Hornady recently introduced two new rifle cartridges--the .300 and .338 Ruger Compact Magnums (RCM)--and Ruger introduced two new Model 77 Hawkeye rifles chambered for these short magnum rounds. So I purchased a stainless, synthetic model chambered in .338 RCM and decided Winchester's 760 propellant might be suitable considering the relative case-to-bore capacity (expansion ratio). Later, I ordered a matte-blue, walnut-stocked Hawkeye in .300 RCM. At the same time, Alliant was touting its new Reloder 17 as an ideal short-magnum propellant. Its burn rate was advertised as similar to 4350, so I ordered a couple pounds in order to develop some test loads in the .300 RCM. But there was no Reloder 17 load data.
The experience obtained while testing handloads in the new RCM cartridges meant I could also safely develop Reloder 17 handloads using the V/C ratio method. I considered the following factors before assembling any handloads: Always start with a reduced charge, but not too low--at least 85-percent case capacity. For larger cases, you can increase the powder charge in 1/2-grain increments. Use Magnum primers if you are loading spherical powder (not in this case) and/or if the charge weight exceeds 60 grains (as it does here) to help ensure consistent ignition. Seat the bullet about 0.030 inch off the lands if the resulting cartridge overall length will still fit and reliably feed from the magazine.
|Using The V/C Method |
|Bullet.||Powder (Type)||Powder (Grs.)||Velocity (fps)||V/C Ratio||Dynamic V/C||Dynamic V/C vs. Avg V/C|
|.300 RCM |
|Hornady 165-gr. SST||Reloder 17 ||60.0 ||2830 || 47.2|| --- || --- |
|Hornady 165-gr. SST||Reloder 17 ||60.5 ||2855 || 47.2|| 50 || 1.1 |
|Hornady 165-gr. SST||Reloder 17 ||61.0 ||2892 || 47.4|| 74 || 1.6 |
|Hornady 165-gr. SST||Reloder 17 ||61.5 ||2930 || 47.6|| 76 || 1.6 |
|Hornady 165-gr. SST||Reloder 17 ||62.0 ||2968 || 47.9|| 76 || 1.6 |
|.338 RCM |
|Hornady 200-gr. JSP|| W760 || 59.5 ||2492 || 41.9|| --- || --- |
|Hornady 200-gr. JSP|| W760 || 60.5 ||2555|| 42.2|| 63 || 1.5 |
|Hornady 200-gr. JSP|| W760 || 61.5 ||2599 || 42.3|| 44 || 1.0 |
|Hornady 200-gr. JSP|| W760 || 62.5 || 2645 || 42.3|| 46 || 1.1 |
|Hornady 200-gr. JSP|| W760 || 63.0 || 2680 || 42.5|| 70 || 1.6 |
|Hornady 200-gr. JSP|| W760 || 63.5 || 2707 || 42.6|| 54 || 1.3 |
|Hornady 200-gr. JSP|| W760 || 64.0 || 2723 || 42.5|| 32 || 0.8 |
|Hornady 200-gr. JSP|| W760 || 64.5 || 2752 || 42.7|| 58 || 1.4 |
|Hornady 200-gr. JSP|| W760 || 65.0 || 2781 || 42.8|| 58 || 1.4 |