Although there is not much context, I don't think there can really be much doubt about what this refers to. Normally the left coronary artery (LCA, to which TCI refers here) arises from the aorta, runs for 10-25 mm and then bifurcates into the left circumflex (LCx) and the left anterior descending (LAD) arteries. There is an anatomical anomaly in which LCA is absent and LCx and LAD arise directly from the aorta, from separate ostia. This is known as cañón de escopeta in Spanish and as double barrel in English. Our text is saying that TCI (LCA) is very short in this patient, so short that this is almost a case of double barrel.

'La anomalía más frecuente son los orígenes separados de la DA y CX en la aorta (esto es, la ausencia de TCI, que vulgarmente se llama "cañón de escopeta"), que se da en el 0,4-1% de las personas'
http://gredos.usal.es/jspui/bitstream/10366/83210/1/DAHH_Cas... (p. 25).

'Absent Left Main Trunk (Separate Ostia of Left Anterior Descending and LCX)
Likely the most common coronary anomaly (incidence ~0.47%), separate ostia of the two main branches of the LCA is often referred to as a "double barrel"'
Cardiac Catheterization Handbook, p. 139
https://books.google.es/books?id=7GByCgAAQBAJ&pg=PA139&lpg=P...