Both will significantly affect Pressure. When I started using Quickload I played with these 2 variables and was amazed just how much they play an important role. It's impossible for a reloading manual to know what brass you're using and if you're jamming, jumping, loading mag length etc.
A couple of points:
1) The powder and bullet company manuals' maximum loads are based on SAAMI spec barrels (bore/groove diameters) and a true SAAMI spec chamber. In many cases, this chamber will have considerably less freebore than a typical factory rifle chambered in that cartridge. It used to be said on this forum, and only partly in amusement, that Remington chambered its M700 308s so 'long' that they were either ideally suited to optimal seating depth for the Berger 185gn Juggernaut, or even a bit too long for that bullet. That sort of freebore length drops MVs and pressures substantially. The problem is that in a typical factory rifle you don't know what the manufacturer has given you in terms of chamber dimensions, although the chances of it being true SAAMI spec are very low indeed. (Well, you don't know until you start playing with a COAL gauge and find that your lighter/mid-weight bullets can't be seated in the case and remain near the rifling.)
2) Nearly all manuals list
all components including the case. The small print says that if you deviate from those exact items, the data provided are invalid as far the powder/bullet company are concerned. This can be an issue for some cartridges in particular. Speer warns that 243 Win sees an unusually large range of case capacities by make and that its data apply solely to Winchester brass. Any deviation can cause substantially raised pressures making the data invalid. Speer likewise used Israeli IMI 223 Rem brass in its loads data for many years as it was the thickest walled / lowest capacity commercial brass on the US market, so it was unlikely a handloader would use its data in anything with lower capacities. As you say, the manual compilers cannot know what brass (or primer for that matter which can also have a significant peak pressure effect) the handloader will use, but as I said you're on your own as soon as you change anything.
3) There is a misconception around pressure vs COAL and modelling this with QuickLOAD or GRT. Internal ballistics programs want the COAL that sees the bullet just off the lands as that allied to the case water overflow capacity is the initial chamber combustion chamber volume. In many cases, people seat the bullet to a shorter COAL than that point, for magazine fit or other reasons. Seating a long bullet to 2.800" in a 308 where the chamber defined COAL is say 3.00" won't increase pressures, although it might not leave enough room for the powder charge. If it changes pressures at all, it'll reduce them. When Viht powders were first introduced into the UK market getting on for 45 years ago, the company did a very useful free loading guide on heavy paper that folded up 20 which ways like regional road maps sold in roadside filling stations. It covered much more than just loads for the limited number of cartridges and powders included (all rifle IIRC) and included a pressure barrel measured table of pressures / MVs for a standard 7.62 NATO load using Lapua components and Viht powder in which the bullet was seated progressively deeper, by what increment I can't now remember. Pressures and MVs stayed constant throughout the changes until the final (deepest) bullet seating position/COAL when pressure
dropped. (Note, small capacity pistol cartridge rounds, especially 9X19mmP, display the exact opposite behaviour and there pressures do rise as bullets are seated deeper, potentially dangerously so in some cartridge and loads combinations.)
Incidentally, whilst companies like Sierra use a SAAMI compliant test chamber/barrel, the MVs listed in the tables sometimes come from a factory rifle and barrel, its details listed on the introductory page for the cartridge alongside dimensions and cartridge history, uses and so on. This is because once handloaders started to acquire chronographs, there was almost always a mismatch between the loads table data and the actual results users obtained from their own rifles, the latter being almost invariably lower. This again was due to the mismatch between factory chamber and bore dimensions and the 'tighter' SAAMI or CIP specifications. So the practice of measuring actual maximum safe loads (or to whatever discount from maximum the company decides to adopt) and then retesting MVs in a typical user's rifle was started and is now very common. Not all companies do this and not all tell you what the MVs are obtained in. Sierra does and tells you the rifle. Nosler tells you it was a Wiseman (or whatever) test barrel which means this is unmodified SAAMI spec barrel results. Lyman appears to use both sometimes for a single cartridge as in 7X57mm Mauser where the firearm changes within single tables. Firearms shown in the introduction are 'Universal receiver' ie the lab set-up using a proper pressure test barrel which one has to assume is SAAMI spec, or an actual firearm in this cartridge's case a 29-inch barrel Mauser 95 service rifle. No pressures are listed for the latter, only MVs and the compilers tell you that they don't know what they were loading to pressure-wise, but that the loads were safe in it and similar older models.
