If, as noted in a couple of articles I posted about the US 25th Inf Div yesterday, the area of interest of the bog standard 9-man rifle squad equipped with a suas is 3 to 5 km
Then, the 11 to 17 km 105mm is a Platoon weapon.
1x 105mm will cover the area held under observation by 3x suas.
Think that one through again. Add to your thought process the number of people required, the lack of ability to mass fires over a large front and the terminal effects of the weapons coming in.
Where does that math go if the question shifts from "war gun vs training gun" to "preferred war gun vs acceptable war gun"?
Hypothetical RCA shift
CMBG (x3) = 1x 8 gun SP battery, 1x 8 gun 777 battery, Plus 6 of each for schools etc. Plus 8 gun SP battery for Latvia
To accomplish this we'd need 24+6+8 = 38 new SP guns. Dollars being equal, do you take the 38 K9's/ Archers and just look after the Regs, or the 76 (being conservative) Atmos 2000/ Caesar that equips the regs PLUS 6x 6 gun Pres Batteries enabling each CMBG to field a 30/70 SP Bn?
But that’s not really a debate we’re having. I also don’t think mixed regiments of M777s and SPH makes any sense, nor does maintaining 2 gun batteries.
Your second solution is a better option, with the 3rd gun batteries being reservists - where that makes sense. More guns, of a single type.
The situation of a mixed regiment is not unusual and happens when you have a unit supporting an organization of mixed requirements. Back in the 90s Canadian RegF artillery regiments consisted of two M109 batteries and one LG1 battery to support a brigade of 2 mech bns and 1 light bn. It's generally not a preferred solution for the brigade as a whole in combat, but it does make sense when all you are generating is a battalion for Bosnia while retaining a stronger capability in your hip pocket.
Our operational focus for the last twenty years, at least, has concerned itself with force generating battle groups rather than brigades or divisions, and in that scenario you can get away with a lot of deviations from doctrine and go with ad hocery.
The issue should always be: what is the largest organization that your country anticipates it may need to deploy in a worst case situation and then build your doctrine and an appropriate force structure to meet that. You can always go with less in a less serious situation. If you underbuild the force structure in the first place then you risk disaster.
I think that with the Latvia mission, being for a brigade, dictates that our doctrine and structure should be, at a minimum, to build operational brigades that can operate in a division in a high intensity conflict. Canadian artillery doctrine generally contemplates a brigade with four manoeuvre units, 1 armoured, 3 infantry that generally form three combined arms battle groups. That means a 3 battery CS regiment with additional GS resources coming from a division. Notwithstanding our actual brigade structure (which has more to do with generating battle groups rather than fighting as a brigade, and which only has two LAV bns it is capable of equipping) in our doctrine we anticipate fighting at the brigade level with a unitary brigade, generally a mech one. We haven't given much thought in the past thirty years or so fighting a light brigade like we did in the 1970s. Accordingly the regiment should have the same type of guns across its three CS batteries albeit if a GS firing unit is added to it, it could be a different system to meet that requirement.
So to answer your question, IMHO the question is never what do you give the RegF and what do you give to the ResF, but what weapons systems and establishments do you need to arm your worst case deployable doctrinal establishment and then supply it with the weapon systems needed to arm it and to sustain that force through training and casualty replacement. Within that overall requirement you decide how many RegF and ResF you need to deploy it in the most likely scenarios and then assign and train those forces appropriately.
My own analysis of what Canada needs, and can provide out of its current manpower levels, is set out in the diagram I posted above. Effectively it calls for three SP regiments to support the mechanized brigades of 1 Div and three towed regiments to support the lighter force that makes up 2 Div (albeit I would also consider, maybe even prefer, having the 2 Div arty bde with two M777 regiments and one SP regiment))
@markppcli has it right. The only place that he and I diverge is that he prefers a three battery regiment where two of the batteries are RegF (which equates to a higher level of readiness) and I'm prepared to go with a three battery regiment of which only one is RegF. IMHO that's enough to sustain the force during peacetime - when the need for deployed gunners is low - but still generate a full regiment during war time. Regardless of that difference, the unit must be provided with the same weapon system for both RegF and ResF elements. I'm accepting more risk in order to build a larger artillery organization out of current RegF arty PYs.
One last comment about Canada's ad hoc arty structures. If you look at an American arty bn all that you will see is a HQ battery and three gun batteries while Canada has a HQ battery, two gun batteries, an OP battery and an STA battery. The limited numbers of gun batteries is purely an available PY issue resulting from giving up a gun battery to create the STA battery and more heavily manned FSCCs and FOO detachments. It has nothing to do with doctrine. Similarly, the Canadian four gun battery is entirely due to the fact that M777s have ten man detachments v the seven man ones for the previous systems. It's not a doctrine thing. I think we have too many batteries and need to reorganize.
When the artillery reorganized subsequent to 2005 it had to do so without any additional RegF positions being allocated. Canada has been thinking at the battery level for several decades now. That's because in 2005 the army declared that artillery regiments were merely force generators of composite batteries put together in building block fashion from several diverse troops. Regiments are no longer "force employers". When you think primarily at the battery level weird things happen at the regimental one. Doctrine still exists (albeit it should change with what we're learning in Ukraine) however the funding, personnel and equipment needed at the regimental level to fully organize and train as per doctrine, simply isn't there.
Ukraine has pulled us out of that thought process and folks are working hard to get back on track while looking at the future. It's anyone's guess as to how many resources will be made available to do that.
. 
