Thoughts on tank gun calibre, and why a mix of 105 mm and 130 mm w/ATGW support might be a consideration for next gen AFV families. This is more offering alternative novel ideas for consideration than my own preference or opinion. #tanktwitter / #miltwitter 
Most discussion around tanks focus on increasing penetration via more advanced ammunition natures for 120 and arguments around whether, and how, to integrate 130 or even 140 in a viable design. #MGCS, DLP and so on all looking at these issues.
Greater pen is certainly a key requirement - the primary role of a tank is exploitation via kinetic defeat of enemy tanks and complete overmatch of lesser AFV. In the direct fire gun vs armour race, armour is slightly ahead at present and 120 mm is close to its theoretical limits
To maintain overmatch and ensure an enduring ability to continue to enhance performance the next gen of tank needs to use 130 or 140mm, lest it just be an upgraded current gen vehicle. Step change enabler are required and that looks to be 130 mm for direct fire
However this is all in the top trumps world of 'my best vs your best' in isolation from effects other than total penetration and ignoring the broader picture. Most 120 rounds are more than capable of dealing with contemporary threat armour.
If nothing else, we must avoid this common top trumps approach - not getting 100% pen does not mean you failed. Any hit can damage or destroy systems, cause external and internal damage and failures, spalling etc. 'Soft' effects on crew not to be dismissed.
By example, a 57mm round from an AU-220M Baikal turret wont penetrate M1 frontal armour. But at 2 per second with 80 ready rounds it would be ludicrous to claim it was not effective or has anything but hugely damaging effects. With HE/AP mix this would be devastating
Also consider that comparisons are usually best pen vs best frontal armour at notional 1,500 m. Future war will be congested, often urban, and close range, <500 m likely and at other than front-to-front presentation. At that range even 90 or 105 mm is hitting w/tremendous energy
Aside: ATGM also continues to be a valuable additional capability offering smaller, faster, more holistically survivable platforms for anti-armour and anti-structure engagement. I dont advocate death of the tank, but fleet of highly mobile ATGM vehs would be valuable.
Here a Sandcat with Spike launcher. Small, mobile and very much speaking to UKs CF(L)35 vision for dispersed "high mobility, organic, direct fire support of infantry and general armoured operations" using ATGM vehicles over tanks. MRVP/JLTV/Serval platform would be well suited.
Back to direct fire. So the argument goes, most of a tanks anticipated effects in contemporary battlefield are in secondary target sets - light/medium armour, logistics and light vehs, bunkers and structures, anti-personnel and static overwatch.
105mm offers clear overmatch against these targets but at lower cost (gun & round), more rounds for same volume or less volume for same round count, lower collateral damage owing to smaller explosive content and enables smaller, lighter and more deployable platforms.
Latest 105 developments continue to push capability. John Cockerill presented at IAV in January showing 105 APFSDS-T pushing 580 mm RHA penetration and noted successful firing of M900 depleted uranium rounds (probably connected statistics).
105 barrel launched missiles like LAHAT and Falarick also offer good capability, again at smaller volume and weight than 120 equivalents. Russia uses these extensively where the West shows no real interest, preferring dismounted ATGW with few ATGW vehicle examples either.
In this broader context the main benefit of 120 - its larger and more powerful APFSDS rounds - is not of great utility. The smaller 105 calibre still offers overmatch against all but the heaviest armoured targets and brings the above benefits.
Discussion point therefore is for notion of mixed fleets in future. Small number of 130mm tanks with larger number of 105mm support tanks, augmented with small ATGM vehicles as a means to dispersed and highly mobile organic longer-range anti-armour/anti-structure effects.
Result is a lighter, smaller, more mobile and deployable force with a lower logistical footprint. Remains capable of engaging peer armour but optimises for most of the time where you dont have massed tanks before you and the negatives of big tanks outweigh benefits.
Organise as desired. Options could include mixed tank Pls w/1-2x 130mm and 2-3x 105mm, or alternatively dedicated 'tank hunter' Pls of 130mm tanks, with majority tank Pls being 105mm-armed formations. More likely a flexible formation built to the specific threat
Russia looked at model for Armata w/combat units of 2xT-14 & 3xT-15. UK/US could consider similar approach, like SBCT organisation with 3x armd inf Pls (IFV w/40-50mm) supported by a 105-tank Pl with reinforced 130 Pl as a shared Bn asset to support where heavy armour identified.
