What
@Pirate69 writes above about throat/leade length (ie 'freebore') is 100% correct. Although well-known by many competition shooters and manufacturers for a long time, it still causes misunderstandings and confusion even on forums like this. A recent thread on 223 Rem FTR loads / pressures with Viht N150 and 90gn bullets showed this clearly. The OP's chamber was throated exceptionally long to see the chosen bullet seated ideally in the neck giving a COAL way beyond SAAMI and allowing (not just allowing, but needing) much heavier charges than those shown in factory reloading loads-tables. As this saw charges over 2gn higher than those in Viht's data, several contributors insisted this had to be an over-pressure dangerous combination attaching images of the relevant table in their posts to 'prove' the point.
Another and even more striking example of this effect is to compare data for the 6.5-284
Winchester wildcat with that for the 6.5-284
Norma. Most people will assume they're the same thing, but strictly speaking the former retains the parent cartridge's 2.8-inch or thereabouts COAL and appropriate length freebore. When Norma adopted the wildcat and changed it to optimise performance, the freebore and hence COAL were massively increased and charges / MVs also rose (and not by small amounts either). In practice, being a wildcat, the Winchester version could of course have any variation of freebore the shooter and his gunsmith decided on to suit the chosen bullet and purpose.
However, what even many of those who understand the principle involved here (ie making changes to the effective combustion chamber volume through chamber throating length) and its effect on pressure often become tied up on is the
actual COAL figure the shooter seats the bullet to, as opposed to what the chamber allows. So people tell you that seating the bullet deeper increases pressures, irrespective. This applies to low capacity pistol cartridges, a classic example being 9mmP, but not to much higher capacity rifle numbers employing vastly slower burning powders. Yet, GRT and QuickLOAD apparently confirm this view - reduce COAL and pressures rise, so charges must be reduced. Whilst this is correct where the COAL change reflects the actual chamber and the bullet's relationship to the lands to keep it at them or just 'out', it overstates pressures when the cartridge is loaded shorter than is optimal for the chamber. Take popular short-action numbers like 308 Win, 260 Rem and suchlike. The shooter wants to single-load long heavy bullets at the optimal COAL and has his gunsmith chamber the barrel accordingly. Say the bullet seated optimally for the chamber gives a COAL of 3-inches or even a tad more. No issue, and assisted by GRT/QuickLOAD, the shooter works up heavy but safe charges of a suitable powder on the range taking note of actual MVs and the appearance or otherwise of pressure signs. However, the user also wants magazine operation for a different application and this restricts the COAL to 2.8-inches or even a little less in order to obtain reliable feed. The chamber is still the same but bullets are now making a 200+ thou' jump to the rifling instead of 15 or 20. GRT/QL now show pressures have increased massively at the new COAL. But have they? I'd argue not - in fact in some instances they can reduce. As peak chamber pressure is normally produced after the bullet has left the case (in fact has travelled an inch or two down the rifled section), the original increased effective chamber volume (from the long throat/leade) still applies and so do the internal ballistics. Others argue that this isn't the case as increasing fill-ratio in itself increases pressure. (Bullet companies such as Nosler say otherwise though and Nosler provides maximum charges in some combinations that run at 110-115% fill-ratios, ie heavily compressed, more so than I'd ever be happy with.) I'm assuming in these hypothetical cases, that the same powder is suitable and charges can be physically accommodated at all COALs, whilst in practice seating bullets deeper often produces excessive charge compression requiring a charge weight reduction.
The situation I outline (bullet seated deeper than optimal for the chamber form) and thereby reducing pressures / MVs is very common in practice of course, whether knowingly or otherwise. An example of the former situation is 223 in custom AR-15 XTC competition rifles whose chambers are throated for and incorporate a longer / gentler leade angle to suit the 80gn SMK or similar bullet seated out to 2.550-inch or so COAL for single-loading, but shorter/blunter 68-77gn bullets are seated to 2.25/6-inches magazine length for the short-distance rapid fire stages. Assuming the latter combinations would physically accept heavier charges than those provided for standard 223 Rem loads in a short-freebore SAAMI compliant chamber, I'd argue that they could be safely employed in the long-freebore chamber. Another, even more prevalent condition, is where for whatever reasons, factory rifle manufacturers chamber their products with way longer freebores than those in the SAAMI chamber spec. Some years back, it was regularly said in Accurate Shooter Forum posts that out of the box Remington 700s in 308 Win were so long-throated that the chambers were about ideal for the longish Berger 185gn Juggernaut seated shallow.
Many years ago (1980s), Viht's loading data pamphlet (a single large sheet with multiple folds like you'd find in contemporary road maps sold in filling stations) had a little section and table on the effects of bullet seating depth on MVs/pressure. It took a standard Lapua 7.62 Nato factory loading and seated the bullet progressively deeper, thereby reducing the available case volume, in IIRC 30-thou' steps. The first three deeper seatings had no effect on either measured chamber pressure or MV; the final deepest setting saw a slightly increased MV accompanied by a small PMax
reduction. The hypothesis often given for this phenomenon is that a very long bullet jump before hitting the lands gives the bullet 'a run at them' thereby reducing friction / resistance to further movement compared to that when seated at the lands or just out.
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