Created on 19th May 2009

LAURIE HOLLAND takes a look at the primer and excess pressure symptoms such as cratering

THE FINAL component before we get on to bullets and propellants is the primer, the small rifle type used here. We have a choice of six makes, some providing standard, magnum and benchrest types. Standard models can develop excess-pressure symptoms too readily, in particular ‘cratering', in this cartridge. In its extreme form this phenomenon gives pierced primers, a small metal disc from inside the ‘crater' blown back into the bolt each time it occurs, escaping gas quickly eroding the firing pin-tip and bolt-face too - very undesirable. ‘Cratering' starts in some .223R rifles when charges are well below top loads, and the chronograph says MVs are modest, therefore it is reasonable to assume pressures are also below the allowed level. I didn't have this problem with the cartridge in a slow-twist rifle loading 52-55gn bullets, but it became more prevalent with each increase in bullet weight. What are the causes other than excess pressure? The firing pin-tip fits the bolt-face loosely in some factory rifles allowing premature ‘cratering', especially if the mainspring is weak. ARs, including our straight-pulls, use a light free-floating firing pin that is held against the primer cup solely by the trigger assembly hammer and its spring, a setup that also sees some primer cup extrusion at modest pressures.

I discovered early on that I could not use the Remington 6½, CCI-400 or PMC SR with any heavy bullet load that provided standard velocities. The sole standard primer that worked well was Winchester's WSR, but this was the older (silver) type, and it's said the current (brass colour) version has a thin cup. The answer is magnum or BR types with their thicker, stronger cups - I use Remington's 7½BR and the Brazilian CBC Magtech 7½. As usual, I seated them with a Lee Auto-Prime hand tool which gives lots of ‘feel'. It's important for straight-pull owners to ensure every primer is seated fully to avoid the fast-moving bolt hitting a proud example and risking a ‘slam-fire'.

Bullets and Seating

I tried six bullets - the 75gn Hornady A-Max, 77gn Sierra Match King (SMK), and 80gn models from Berger, Hornady, Nosler and Sierra. The 77gn SMK is the odd man out, designed to be seated at the standard .223R cartridge overall length (COL) of 2.25-2.26 for magazine operation. Its length is kept down compared to its 80gn sibling by designing in a blunter (smaller radius) shoulder-nose section and shorter boat-tail at the cost of reduced BC (0.372 compared to 0.420). The 80s (plus the 75gn Hornady A-Max) are long designs that have to be seated well out for a longer COL, hence single-loaded in the rifle. Their seating depth has an important effect on the available powder capacity and performance, and crucially on the pressures generated by any particular powder charge (Table 1).

The 80gn SMK has a heavy jacket, which is one reason why it has a reputation as being COL/jump tolerant. In theory, the Nosler with its thinner J4 jacket will be more temperamental, likewise the 75gn A-Max, but I haven't found this. The last-named has given me excellent results, and can be driven 100fps faster than the 80s without producing pressure problems. I found the two longest designs - the 80gn Hornady A-Max and Berger VLD - fussier, maybe due to the ‘VLD-factor', or they might perform better in a faster (7) twist barrel.

How to determine seating depth? This is easy for the 77s which are seated to 2.25" or fractionally longer, although even here one should check the bullet is not jammed into the rifling in a short-throated chamber. The best solution is a COL gauge, the most popular being the Hornady (formerly Stoney-Point) OAL Gauge, with the appropriate modified cartridge case alongside callipers and a ‘bullet-comparator'. Bullets were set around 10 thou off the rifling, other than the Berger VLD which was seated so that it would just ‘kiss' the leade, but some of my loads had a considerable amount of jump by the time I got around to this series of tests. When the rifle was new, this setting gave a COL just under 2.44" with the 80gn SMK, a typical result for a .223 reamer for fast-twist barrelled match rifles, and considerably shorter than the military 5.56mm chamber or that of the Colt AR15A2 Match Target Competition HBAR rifle used by most American loading manuals for their heavy-bullet data. With various throat configurations used in fast twist .223R rifles, the only way you know what you've got in order to set the seating die correctly is through measurement.

Rifle, Testing and Benchmarks

All groups listed in the results table were shot off a concrete bench with a rear-bag under the buttstock. I initially used a heavy Sinclair front-rest with a general-purpose top; later I switched to a Versa-Pod bipod which reduced a tendency to string shots vertically, the SSR's round forend tube not ideal for a rest and bag setup. All cartridges were single-loaded, even those 77gn SMK combinations seated to magazine COL, to keep conditions consistent. I facilitated single-loading by replacing the AR15 magazine with an ‘Original Bob Sled'. This provided a deep feed groove, while still operating the rifle's hold-open facility on opening the bolt.

A straight-pull tells you clearly when pressures are becoming significant by the amount of effort needed to ‘unstick' the fired case and get the bolt assembly moving. The ambidextrous SSR allows a consistent, easy bench-shooting regime which allowed me to keep the butt in my shoulder for a fast and smooth shooting movement that delivered good groups. I used a CED Millennium chronograph to record MVs and calculate an average, extreme spread and SD (standard deviation) for each five-round test batch.

When I first tried the 77gn SMK, I discovered that seating them just short of the rifling gave no accuracy benefit over magazine length. So, when I got around to a more formal programme, I simply adjusted the seater die to give 2.255 COL.

Throat wear with the .223 is mild, generally reckoned to run around half that of .308W, which is itself no barrel-burner, around 10 thou erosion per 1000 rounds compared to 20 being an estimate I've seen in print a few times. I expected the start of the rifling to have moved forward around 60 thou, so got a surprise when it turned out to be nearly double that. (Loading heavy bullets increases throat erosion substantially for reasons that I cannot cover here, and this will account for much extra wear.) Seating the 80gn SMK just short of the rifling now gave a COL of 2.560" which saw the bullet barely gripped by the case-neck. Conversely, I loaded the Berger VLDs and two Hornady A-Max designs with their secant ogives almost on the rifling. As noted, the 75gn A-Max continued to perform well, but I had less luck with the other two. Another reason for this could be the issue of throat condition - a worn, rough run-in to the rifling not suited to these designs.

Before starting new load development with the longer bullets, I thought I'd get some benchmarks, shooting and averaging three-group strings with a small selection of my older loads still at their original COL settings. As can be seen in Table 3, averages were still presentable, especially as I ‘fluffed' my second 80gn SMK/Re15 group. Incidentally, the 73gn Berger/H4895 combination's 0.46" was an average of four groups and this mild load (2,765fps) was kept to 2.26" COL for magazine operation, the bullet being a long-established ‘LTB' design, now renamed ‘Match BT', and superbly accurate out to 600yd despite a long jump in the chamber. It also works in 9T barrels, so this is one effective all-rounder!

Powders and Results

With nearly all loads-data for the cartridge coming from the USA, sources concentrate on lighter bullet loads for the gopher-hunter, or in target shooting, powders and charges suitable for semi-auto operation in 20-barrel AR15s. (Although the 600yd National Match stage sees over-length 80gn-bullet rounds single-loaded, gas operation remains mandatory, the action cycling to the locked-open position, this limiting loads and pressures.) ‘Standard loads' for a few powders are used by the majority of competitors - 24.0 to 24.5gn Vihtavuori N140, Hodgdon Varget and Alliant Reloder 15 for 77/80gn bullets.

Maximum published loads in reloading manuals are obviously kept down given the likelihood they'll be used in semi-auto rifles, the giveaway being the near universal use of 20" 7T AR15 barrels to obtain MVs. QuickLOAD estimates PMax in the range of 47,518 to 53,627 psi for Sierra's AR15 data. Most loads are around halfway between these values, some 5,000 psi under the SAAMI maximum. Another factor that keeps published loads down is brass variations with so much heavy military stuff around, and this may be why Vihtavuori's maxima are down on even US-sourced material, pressure testing carried out before the Lapua Match case with its increased capacity was introduced. There is a final factor that American shooters must take into account - ambient temperature. With this little case, a safe load worked up in the New England spring can generate enough additional pressure in Camp Perry's August heat to see the primer blown out of the case into the AR's receiver as the action cycles. This is a less pressing issue here, but Bob Clark recommends Varget, one of Hodgdon's temperature-insensitive ‘Extreme' powders, as he has seen PR competitors struggle to open SSR-15 bolts on hot summer days.

It's therefore reasonable to assume turn-bolt type rifles can use slightly higher maximum charges than those listed, as shown by Sierra's ‘bolt-action' load-map, some maximum charges increasing a full grain or more - a lot in .223/80. I assumed my SSR would accept top loads between the two Sierra data sets. The issue in a manually operated AR is not bolt-lug/action strength, but how much effort is required to ‘unstick' fired cases? In the event, only a very few of my test batches produced even modest resistance. As I'd had little experience with the 77gn SMK, I stuck to Sierra's maxima and found them very mild, MVs around 2,690 fps the norm. Having loaded the 80s before with various powders, I went a little higher than some of the bullet-makers' top loads in order to approach 2,800 fps. I ran potential loads through QuickLOAD to find charge weights that should produce 50,000-52,000 psi. I weighed all charges on a set of RCBS 10-10 scales, and the usual range over five batches was 1.3-1.5gn, initial 0.4gn steps dropping to 0.3gn, then 0.2gn as they approached the maximum. I'll stress here that you must increase charges in very small steps as you approach maximum loads - 0.2gn is about right; the half-grain changes that some people use are far too large for this little cartridge, except when moving up from a low ‘starting load'.

The results in Tables 3 and 4 speak for themselves, but I'll make some observations. The 77gn SMK turned out to be remarkably consistent and accurate with all eight powders tried. In fact, out of the 40 groups fired, only one exceeded 1, and the average size on the target for the whole 40 was only 0.61. The best performers in terms of five-batch averages (on or smaller than half-inch) and smallest individual groups (quarter to third-inch) were IMR-4320, Alliant Re15 and Hodgdon H4895. Interestingly, this trio's top charges all produced MVs within 7fps of each other at just under 2,700fps.

The 75gn A-Max also gave best results with H4895 and Reloder 15, the latter at relatively low velocities. Highest MVs came with the two Accurate Arms ball powders, 24.5gn of 2460 giving a stunning 2,990fps, but group size rose in line with MV. Even though I didn't encounter excess-pressure signs on a warm (20ºC) day, I don't recommend going above 24.0gn, this still giving over 2,900fps. Both it and 2520 left a difficult-to-shift hard fouling coating on case-necks - that alone would make me avoid them!

When I got around to the 80gn SMK, my initial top load of 24.0gn with Viht N140 and Alliant Reloder 15 both gave excellent groups, but rather low MVs around 2,700fps. As there were no pressure problems, a second batch of each combination was loaded rising to 24.5gn in very small steps. N140 continued to give small groups until 24.3gn, but then opened out considerably, this charge weight averaging 2,753fps with a reasonable spread by .223 standards (32fps). Reloder 15 gave excellent results right up to 24.5gn at which level I was getting 2,744fps but with a larger 60fps extreme spread. The other ‘star' with this bullet was my old favourite in the cartridge, Hodgdon Varget. Having long used charges in the 23.5-24gn bracket without problems, I worked up to 24.5gn this time around, this load giving me a third of an inch group at a respectable 2,785fps and 38fps ES. Viht N540 gave good velocities, but large groups - these two characteristics probably related! The ‘find' with this bullet was slower-burning Viht N150, ideal for my SSR's long (26) barrel. I moved onto the Sierra starting at 25.9gn and going up in 0.1gn steps. Despite poor range conditions, the top load of 26.3gn gave a 0.44 group at 2,907fps MV with an SD of 10fps. Bolt opening was just starting to stick, so this may have been ‘pushing' the load a little too far for an SSR in hot weather conditions. The 80gn Nosler - one of my favourites - gave its usual sterling performance with Reloder 15 and Varget, as well as the first go with Viht N150, the first named producing the smallest groups. The 80gn A-Max was remarkably consistent with five powders all running in the half to full-inch range, Hodgdon Varget giving the best performance and reasonable MVs finishing up at 2,777fps with the top load of 24.5gn. Again, bolt opening was just getting sticky, so I would stop at 24.2 or 24.3gn with this combination in my rifle.

Having now covered the three main bullet weight groupings over the years, I'll be experimenting further with N150 and its high-energy N550 double-base partner; an 80gn Match BT (tangent ogive) bullet is expected soon from Berger which will surely be worth trying.


These components and loads performed safely in the author's rifle. This cannot be guaranteed for other handloaders and rifles. Good handloading procedures should be used at all times, working loads up from starting levels at least 10% below the maximum loads listed in the results table and looking for signs of excessive pressure or other problems. (See the notes to the results tables.) Warnings about case make/capacity, primer type and throating/COL must be heeded, otherwise excessive pressures/blown primers may result.


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